diff --git a/addons/hr/models/resource.py b/addons/hr/models/resource.py
index b59767f6254..d0db7c76b82 100644
--- a/addons/hr/models/resource.py
+++ b/addons/hr/models/resource.py
@@ -1,10 +1,37 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
+from pytz import utc, timezone
+from datetime import datetime
 
 from odoo import fields, models
+from odoo.addons.resource.models.resource import Intervals
 
 
 class ResourceResource(models.Model):
     _inherit = "resource.resource"
 
     user_id = fields.Many2one(copy=False)
+    employee_id = fields.One2many('hr.employee', 'resource_id', domain="[('company_id', '=', company_id)]")
+
+    def _get_calendars_validity_within_period(self, start, end, default_company=None):
+        """
+            Returns a dict of dict with resource's id as first key and resource's calendar as secondary key
+            The value is the validity interval of the calendar for the given resource.
+
+            The validity interval of the employee resource calendar is the lifetime of the employee, from creation to departure.
+        """
+        assert start.tzinfo and end.tzinfo
+        calendars_within_period_per_resource = super()._get_calendars_validity_within_period(start, end, default_company=default_company)
+        for resource in self:
+            if not resource.employee_id:
+                continue
+            create_date = max(start, utc.localize(resource.employee_id.create_date))
+            if resource.employee_id.departure_date and resource.employee_id.departure_date <= end.date():
+                departure_datetime = timezone(resource.tz).localize(datetime.combine(resource.employee_id.departure_date, datetime.max.time()))
+                departure_datetime = min(departure_datetime, end)
+            else:
+                departure_datetime = end
+            interval = Intervals([(create_date, departure_datetime, self.env['resource.calendar.attendance'])])
+            for calendar in calendars_within_period_per_resource[resource.id]:
+                calendars_within_period_per_resource[resource.id][calendar] &= interval
+        return calendars_within_period_per_resource
diff --git a/addons/hr/tests/__init__.py b/addons/hr/tests/__init__.py
index c46b7cc336f..17a4487628d 100644
--- a/addons/hr/tests/__init__.py
+++ b/addons/hr/tests/__init__.py
@@ -6,3 +6,4 @@ from . import test_channel
 from . import test_hr_flow
 from . import test_self_user_access
 from . import test_multi_company
+from . import test_resource
diff --git a/addons/hr/tests/test_resource.py b/addons/hr/tests/test_resource.py
new file mode 100644
index 00000000000..490fe343694
--- /dev/null
+++ b/addons/hr/tests/test_resource.py
@@ -0,0 +1,86 @@
+# -*- coding: utf-8 -*-
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+from datetime import datetime
+from pytz import timezone, utc
+
+from odoo.addons.resource.models.resource import Intervals, sum_intervals
+
+from .common import TestHrCommon
+
+
+class TestResource(TestHrCommon):
+
+    @classmethod
+    def setUpClass(cls):
+        super(TestResource, cls).setUpClass()
+        cls.calendar_40h = cls.env['resource.calendar'].create({'name': 'Default calendar'})
+        cls.employee_niv = cls.env['hr.employee'].create({
+            'name': 'Sharlene Rhodes',
+            'departure_date': '2022-06-01',
+            'resource_calendar_id': cls.calendar_40h.id,
+        })
+        cls.employee_niv_create_date = '2021-01-01 10:00:00'
+        cls.env.cr.execute("UPDATE hr_employee SET create_date=%s WHERE id=%s",
+                           (cls.employee_niv_create_date, cls.employee_niv.id))
+
+    def test_calendars_validity_within_period_default(self):
+        calendars = self.employee_niv.resource_id._get_calendars_validity_within_period(
+            utc.localize(datetime(2021, 7, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 7, 30, 17, 0, 0)),
+        )
+        interval = Intervals([(
+            utc.localize(datetime(2021, 7, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 7, 30, 17, 0, 0)),
+            self.env['resource.calendar.attendance']
+        )])
+
+        self.assertEqual(1, len(calendars), "The dict returned by calendars validity should only have 1 entry")
+        self.assertEqual(1, len(calendars[self.employee_niv.resource_id.id]), "Niv should only have one calendar")
+        niv_entry = calendars[self.employee_niv.resource_id.id]
+        niv_calendar = next(iter(niv_entry))
+        self.assertEqual(niv_calendar, self.calendar_40h, "It should be Niv's Calendar")
+        self.assertFalse(niv_entry[niv_calendar] - interval, "Interval should cover all calendar's validity")
+        self.assertFalse(interval - niv_entry[niv_calendar], "Calendar validity should cover all interval")
+
+    def test_calendars_validity_within_period_creation(self):
+        calendars = self.employee_niv.resource_id._get_calendars_validity_within_period(
+            utc.localize(datetime(2020, 12, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 1, 31, 17, 0, 0)),
+        )
+        interval = Intervals([(
+            utc.localize(datetime(2021, 1, 1, 10, 0, 0)),
+            utc.localize(datetime(2021, 1, 31, 17, 0, 0)),
+            self.env['resource.calendar.attendance']
+        )])
+        niv_entry = calendars[self.employee_niv.resource_id.id]
+        self.assertFalse(niv_entry[self.calendar_40h] - interval, "Interval should cover all calendar's validity")
+        self.assertFalse(interval - niv_entry[self.calendar_40h], "Calendar validity should cover all interval")
+
+    def test_calendars_validity_within_period_before_creation(self):
+        calendars = self.employee_niv.resource_id._get_calendars_validity_within_period(
+            utc.localize(datetime(2020, 12, 1, 8, 0, 0)),
+            utc.localize(datetime(2020, 12, 31, 17, 0, 0)),
+        )
+        niv_entry = calendars[self.employee_niv.resource_id.id]
+        self.assertFalse(niv_entry[self.calendar_40h], "Interval should be empty")
+
+    def test_calendars_validity_within_period_before_departure(self):
+        calendars = self.employee_niv.resource_id._get_calendars_validity_within_period(
+            utc.localize(datetime(2022, 5, 1, 8, 0, 0)),
+            utc.localize(datetime(2022, 6, 30, 17, 0, 0)),
+        )
+        interval = Intervals([(
+            utc.localize(datetime(2022, 5, 1, 8, 0, 0)),
+            timezone(self.employee_niv.tz).localize(datetime(2022, 6, 1, 23, 59, 59, 999999)),
+            self.env['resource.calendar.attendance']
+        )])
+        niv_entry = calendars[self.employee_niv.resource_id.id]
+        self.assertFalse(niv_entry[self.calendar_40h] - interval, "Interval should cover all calendar's validity")
+        self.assertFalse(interval - niv_entry[self.calendar_40h], "Calendar validity should cover all interval")
+
+    def test_get_valid_work_intervals(self):
+        start = utc.localize(datetime(2022, 5, 29, 0, 0, 0))
+        end = utc.localize(datetime(2022, 6, 4, 23, 59, 59))
+        work_intervals, _ = self.employee_niv.resource_id._get_valid_work_intervals(start, end)
+        sum_work_intervals = sum_intervals(work_intervals[self.employee_niv.resource_id.id])
+        self.assertEqual(24, sum_work_intervals, "Sum of the work intervals for the resource niv should be 24h as he left his work after wednesday.")
diff --git a/addons/hr_contract/models/__init__.py b/addons/hr_contract/models/__init__.py
index ce5e380e01d..0c4bf551ae3 100644
--- a/addons/hr_contract/models/__init__.py
+++ b/addons/hr_contract/models/__init__.py
@@ -6,4 +6,5 @@ from . import hr_employee_public
 from . import hr_contract
 from . import res_users
 from . import resource
+from . import resource_resource
 from . import hr_contract_type
diff --git a/addons/hr_contract/models/resource.py b/addons/hr_contract/models/resource.py
index ad4079e2f65..ba000c4de18 100644
--- a/addons/hr_contract/models/resource.py
+++ b/addons/hr_contract/models/resource.py
@@ -1,6 +1,5 @@
 # -*- coding:utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
-
 from datetime import datetime
 
 from odoo import fields, models
diff --git a/addons/hr_contract/models/resource_resource.py b/addons/hr_contract/models/resource_resource.py
new file mode 100644
index 00000000000..5ac7e078eb2
--- /dev/null
+++ b/addons/hr_contract/models/resource_resource.py
@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+from collections import defaultdict
+from datetime import datetime
+from pytz import timezone
+
+from odoo import models
+from odoo.addons.resource.models.resource import Intervals
+
+class ResourceResource(models.Model):
+    _inherit = 'resource.resource'
+
+    def _get_calendars_validity_within_period(self, start, end, default_company=None):
+        assert start.tzinfo and end.tzinfo
+        if not self:
+            return super()._get_calendars_validity_within_period(start, end, default_company=default_company)
+        calendars_within_period_per_resource = defaultdict(lambda: defaultdict(Intervals))  # keys are [resource id:integer][calendar:self.env['resource.calendar']]
+        resource_without_contract = self.filtered(lambda r: not r.employee_id or r.employee_id.employee_type not in ['employee', 'student'])
+        if resource_without_contract:
+            calendars_within_period_per_resource.update(
+                super(ResourceResource, resource_without_contract)._get_calendars_validity_within_period(start, end, default_company=default_company)
+            )
+        resource_with_contract = self - resource_without_contract
+        if not resource_with_contract:
+            return calendars_within_period_per_resource
+        timezones = {resource.tz for resource in resource_with_contract}
+        date_start = min(start.astimezone(timezone(tz)).date() for tz in timezones)
+        date_end = max(end.astimezone(timezone(tz)).date() for tz in timezones)
+        contracts = resource_with_contract.employee_id._get_contracts(
+            date_start, date_end, states=['open', 'draft', 'close']
+        ).filtered(lambda c: c.state in ['open', 'close'] or c.kanban_state == 'done')
+        for contract in contracts:
+            tz = timezone(contract.employee_id.tz)
+            calendars_within_period_per_resource[contract.employee_id.resource_id.id][contract.resource_calendar_id] |= Intervals([(
+                tz.localize(datetime.combine(contract.date_start, datetime.min.time())) if contract.date_start > start.astimezone(tz).date() else start,
+                tz.localize(datetime.combine(contract.date_end, datetime.max.time())) if contract.date_end and contract.date_end < end.astimezone(tz).date() else end,
+                self.env['resource.calendar.attendance']
+            )])
+        return calendars_within_period_per_resource
diff --git a/addons/hr_contract/tests/__init__.py b/addons/hr_contract/tests/__init__.py
index d798e53356b..6f1137322aa 100644
--- a/addons/hr_contract/tests/__init__.py
+++ b/addons/hr_contract/tests/__init__.py
@@ -4,3 +4,4 @@
 from . import test_auto_status
 from . import test_contract
 from . import test_calendar_sync
+from . import test_resource
diff --git a/addons/hr_contract/tests/test_calendar_sync.py b/addons/hr_contract/tests/test_calendar_sync.py
index cecbe399225..40493ddaf9e 100644
--- a/addons/hr_contract/tests/test_calendar_sync.py
+++ b/addons/hr_contract/tests/test_calendar_sync.py
@@ -66,4 +66,4 @@ class TestContractCalendars(TestContractCommon):
         # Transfer global leaves
         self.calendar_richard.transfer_leaves_to(self.calendar_35h, resources=None, from_date=Date.to_date('2015-11-21'))
 
-        self.assertEqual(leave3.calendar_id, self.calendar_35h, "Global leave should be transfered")
\ No newline at end of file
+        self.assertEqual(leave3.calendar_id, self.calendar_35h, "Global leave should be transfered")
diff --git a/addons/hr_contract/tests/test_resource.py b/addons/hr_contract/tests/test_resource.py
new file mode 100644
index 00000000000..0a6021c6051
--- /dev/null
+++ b/addons/hr_contract/tests/test_resource.py
@@ -0,0 +1,109 @@
+# -*- coding: utf-8 -*-
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+from datetime import date, datetime
+from pytz import utc, timezone
+
+from odoo.addons.resource.models.resource import Intervals, sum_intervals
+from odoo.fields import Date
+
+from .common import TestContractCommon
+
+class TestResource(TestContractCommon):
+
+    @classmethod
+    def setUpClass(cls):
+        super(TestResource, cls).setUpClass()
+        cls.calendar_richard = cls.env['resource.calendar'].create({'name': 'Calendar of Richard'})
+        cls.employee.resource_calendar_id = cls.calendar_richard
+
+        cls.calendar_35h = cls.env['resource.calendar'].create({
+            'name': '35h calendar',
+            'attendance_ids': [
+                (0, 0, {'name': 'Monday Morning', 'dayofweek': '0', 'hour_from': 8, 'hour_to': 12, 'day_period': 'morning'}),
+                (0, 0, {'name': 'Monday Evening', 'dayofweek': '0', 'hour_from': 13, 'hour_to': 16, 'day_period': 'afternoon'}),
+                (0, 0, {'name': 'Tuesday Morning', 'dayofweek': '1', 'hour_from': 8, 'hour_to': 12, 'day_period': 'morning'}),
+                (0, 0, {'name': 'Tuesday Evening', 'dayofweek': '1', 'hour_from': 13, 'hour_to': 16, 'day_period': 'afternoon'}),
+                (0, 0, {'name': 'Wednesday Morning', 'dayofweek': '2', 'hour_from': 8, 'hour_to': 12, 'day_period': 'morning'}),
+                (0, 0, {'name': 'Wednesday Evening', 'dayofweek': '2', 'hour_from': 13, 'hour_to': 16, 'day_period': 'afternoon'}),
+                (0, 0, {'name': 'Thursday Morning', 'dayofweek': '3', 'hour_from': 8, 'hour_to': 12, 'day_period': 'morning'}),
+                (0, 0, {'name': 'Thursday Evening', 'dayofweek': '3', 'hour_from': 13, 'hour_to': 16, 'day_period': 'afternoon'}),
+                (0, 0, {'name': 'Friday Morning', 'dayofweek': '4', 'hour_from': 8, 'hour_to': 12, 'day_period': 'morning'}),
+                (0, 0, {'name': 'Friday Evening', 'dayofweek': '4', 'hour_from': 13, 'hour_to': 16, 'day_period': 'afternoon'})
+            ],
+        })
+        cls.calendar_35h._onchange_hours_per_day()  # update hours/day
+
+        cls.contract_cdd = cls.env['hr.contract'].create({
+            'date_start': Date.to_date('2021-09-01'),
+            'date_end': Date.to_date('2021-10-31'),
+            'name': 'First CDD Contract for Richard',
+            'resource_calendar_id': cls.calendar_35h.id,
+            'wage': 5000.0,
+            'employee_id': cls.employee.id,
+            'state': 'open',
+        })
+        cls.contract_cdi = cls.env['hr.contract'].create({
+            'date_start': Date.to_date('2021-11-01'),
+            'name': 'CDI Contract for Richard',
+            'resource_calendar_id': cls.calendar_richard.id,
+            'wage': 5000.0,
+            'employee_id': cls.employee.id,
+            'state': 'draft',
+            'kanban_state': 'done',
+        })
+
+    def test_calendars_validity_within_period(self):
+        tz = timezone(self.employee.tz)
+        calendars = self.employee.resource_id._get_calendars_validity_within_period(
+            tz.localize(datetime(2021, 10, 1, 0, 0, 0)),
+            tz.localize(datetime(2021, 12, 1, 0, 0, 0)),
+        )
+        interval_35h = Intervals([(
+            tz.localize(datetime(2021, 10, 1, 0, 0, 0)),
+            tz.localize(datetime.combine(date(2021, 10, 31), datetime.max.time())),
+            self.env['resource.calendar.attendance']
+        )])
+        interval_40h = Intervals([(
+            tz.localize(datetime(2021, 11, 1, 0, 0, 0)),
+            tz.localize(datetime(2021, 12, 1, 0, 0, 0)),
+            self.env['resource.calendar.attendance']
+        )])
+
+        self.assertEqual(1, len(calendars), "The dict returned by calendars validity should only have 1 entry")
+        self.assertEqual(2, len(calendars[self.employee.resource_id.id]), "Jean should only have one calendar")
+        richard_entries = calendars[self.employee.resource_id.id]
+        for calendar in richard_entries:
+            self.assertTrue(calendar in (self.calendar_35h | self.calendar_richard), "Each calendar should be listed")
+            if calendar == self.calendar_35h:
+                self.assertFalse(richard_entries[calendar] - interval_35h, "Interval 35h should cover all calendar 35h validity")
+                self.assertFalse(interval_35h - richard_entries[calendar], "Calendar 35h validity should cover all interval 35h")
+            elif calendar == self.calendar_richard:
+                self.assertFalse(richard_entries[calendar] - interval_40h, "Interval 40h should cover all calendar 40h validity")
+                self.assertFalse(interval_40h - richard_entries[calendar], "Calendar 40h validity should cover all interval 40h")
+
+    def test_queries(self):
+        employees_test = self.env['hr.employee'].create([{
+            'name': 'Employee ' + str(i),
+        } for i in range(0, 50)])
+        for emp in employees_test:
+            new_contract = self.contract_cdd.copy()
+            new_contract.employee_id = emp
+            new_contract.state = 'open'
+            new_contract = self.contract_cdi.copy()
+            new_contract.employee_id = emp
+            new_contract.state = 'draft'
+            new_contract.kanban_state = 'done'
+
+        start = utc.localize(datetime(2021, 9, 1, 0, 0, 0))
+        end = utc.localize(datetime(2021, 11, 30, 23, 59, 59))
+        with self.assertQueryCount(14):
+            work_intervals, _ = (employees_test | self.employee).resource_id._get_valid_work_intervals(start, end)
+
+        self.assertEqual(len(work_intervals), 51)
+
+    def test_get_valid_work_intervals(self):
+        start = timezone(self.employee.tz).localize(datetime(2021, 10, 24, 2, 0, 0))
+        end = timezone(self.employee.tz).localize(datetime(2021, 11, 6, 23, 59, 59))
+        work_intervals, _ = self.employee.resource_id._get_valid_work_intervals(start, end)
+        sum_work_intervals = sum_intervals(work_intervals[self.employee.resource_id.id])
+        self.assertEqual(75, sum_work_intervals, "Sum of the work intervals for the employee should be 35h+40h = 75h")
diff --git a/addons/resource/models/resource.py b/addons/resource/models/resource.py
index e1050d4a5a2..9c815aacb1a 100644
--- a/addons/resource/models/resource.py
+++ b/addons/resource/models/resource.py
@@ -138,6 +138,12 @@ class Intervals(object):
 
         return result
 
+def sum_intervals(intervals):
+    """ Sum the intervals duration (unit : hour)"""
+    return sum(
+        (stop - start).total_seconds() / 3600
+        for start, stop, meta in intervals
+    )
 
 class ResourceCalendar(models.Model):
     """ Calendar model for a resource. It has
@@ -382,13 +388,15 @@ class ResourceCalendar(models.Model):
         """ Return the attendance intervals in the given datetime range.
             The returned intervals are expressed in specified tz or in the resource's timezone.
         """
-        self.ensure_one()
-        resources = self.env['resource.resource'] if not resources else resources
         assert start_dt.tzinfo and end_dt.tzinfo
         self.ensure_one()
         combine = datetime.combine
 
-        resources_list = list(resources) + [self.env['resource.resource']]
+        if not resources:
+            resources = self.env['resource.resource']
+            resources_list = [resources]
+        else:
+            resources_list = list(resources) + [self.env['resource.resource']]
         resource_ids = [r.id for r in resources_list]
         domain = domain if domain is not None else []
         domain = expression.AND([domain, [
@@ -472,15 +480,18 @@ class ResourceCalendar(models.Model):
         """ Return the leave intervals in the given datetime range.
             The returned intervals are expressed in specified tz or in the calendar's timezone.
         """
-        resources = self.env['resource.resource'] if not resources else resources
         assert start_dt.tzinfo and end_dt.tzinfo
         self.ensure_one()
 
-        # for the computation, express all datetimes in UTC
-        resources_list = list(resources) + [self.env['resource.resource']]
+        if not resources:
+            resources = self.env['resource.resource']
+            resources_list = [resources]
+        else:
+            resources_list = list(resources) + [self.env['resource.resource']]
         resource_ids = [r.id for r in resources_list]
         if domain is None:
             domain = [('time_type', '=', 'leave')]
+        # for the computation, express all datetimes in UTC
         domain = domain + [
             ('calendar_id', 'in', [False, self.id]),
             ('resource_id', 'in', resource_ids),
@@ -518,7 +529,7 @@ class ResourceCalendar(models.Model):
             resources = self.env['resource.resource']
             resources_list = [resources]
         else:
-            resources_list = list(resources)
+            resources_list = list(resources) + [self.env['resource.resource']]
 
         attendance_intervals = self._attendance_intervals_batch(start_dt, end_dt, resources, tz=tz)
         if compute_leaves:
@@ -588,8 +599,11 @@ class ResourceCalendar(models.Model):
         @return dict with hours of attendance in each day between `from_datetime` and `to_datetime`
         """
         self.ensure_one()
-        resources = self.env['resource.resource'] if not resources else resources
-        resources_list = list(resources) + [self.env['resource.resource']]
+        if not resources:
+            resources = self.env['resource.resource']
+            resources_list = [resources]
+        else:
+            resources_list = list(resources) + [self.env['resource.resource']]
         # total hours per day:  retrieve attendances with one extra day margin,
         # in order to compute the total hours on the first and last days
         from_full = from_datetime - timedelta(days=1)
@@ -994,7 +1008,6 @@ class ResourceResource(models.Model):
             )
         return result
 
-
     def _get_unavailable_intervals(self, start, end):
         """ Compute the intervals during which employee is unavailable with hour granularity between start and end
             Note: this method is used in enterprise (forecast and planning)
@@ -1012,6 +1025,52 @@ class ResourceResource(models.Model):
             resource_mapping.update(resources_unavailable_intervals)
         return resource_mapping
 
+    def _get_calendars_validity_within_period(self, start, end, default_company=None):
+        """ Gets a dict of dict with resource's id as first key and resource's calendar as secondary key
+            The value is the validity interval of the calendar for the given resource.
+
+            Here the validity interval for each calendar is the whole interval but it's meant to be overriden in further modules
+            handling resource's employee contracts.
+        """
+        assert start.tzinfo and end.tzinfo
+        resource_calendars_within_period = defaultdict(lambda: defaultdict(Intervals))  # keys are [resource id:integer][calendar:self.env['resource.calendar']]
+        default_calendar = default_company and default_company.resource_calendar_id or self.env.company.resource_calendar_id
+        if not self:
+            # if no resource, add the company resource calendar.
+            resource_calendars_within_period[False][default_calendar] = Intervals([(start, end, self.env['resource.calendar.attendance'])])
+        for resource in self:
+            calendar = resource.calendar_id or resource.company_id.resource_calendar_id or default_calendar
+            resource_calendars_within_period[resource.id][calendar] = Intervals([(start, end, self.env['resource.calendar.attendance'])])
+        return resource_calendars_within_period
+
+    def _get_valid_work_intervals(self, start, end, calendars=None):
+        """ Gets the valid work intervals of the resource following their calendars between ``start`` and ``end``
+
+            This methods handle the eventuality of a resource having multiple resource calendars, see _get_calendars_validity_within_period method
+            for further explanation.
+        """
+        assert start.tzinfo and end.tzinfo
+        resource_calendar_validity_intervals = {}
+        calendar_resources = defaultdict(lambda: self.env['resource.resource'])
+        resource_work_intervals = defaultdict(Intervals)
+        calendar_work_intervals = dict()
+
+        resource_calendar_validity_intervals = self._get_calendars_validity_within_period(start, end)
+        for resource in self:
+            # For each resource, retrieve its calendar and their validity intervals
+            for calendar in resource_calendar_validity_intervals[resource.id]:
+                calendar_resources[calendar] |= resource
+        for calendar in (calendars or []):
+            calendar_resources[calendar] |= self.env['resource.resource']
+        for calendar, resources in calendar_resources.items():
+            # For each calendar used by the resources, retrieve the work intervals for every resources using it
+            work_intervals_batch = calendar._work_intervals_batch(start, end, resources=resources)
+            for resource in resources:
+                # Make the conjunction between work intervals and calendar validity
+                resource_work_intervals[resource.id] |= work_intervals_batch[resource.id] & resource_calendar_validity_intervals[resource.id][calendar]
+            calendar_work_intervals[calendar.id] = work_intervals_batch[False]
+
+        return resource_work_intervals, calendar_work_intervals
 
 class ResourceCalendarLeaves(models.Model):
     _name = "resource.calendar.leaves"
diff --git a/addons/resource/tests/common.py b/addons/resource/tests/common.py
index d4b19ccd701..ee2dee6dff7 100644
--- a/addons/resource/tests/common.py
+++ b/addons/resource/tests/common.py
@@ -6,8 +6,9 @@ from odoo.tests.common import TransactionCase
 
 class TestResourceCommon(TransactionCase):
 
-    def _define_calendar(self, name, attendances, tz):
-        return self.env['resource.calendar'].create({
+    @classmethod
+    def _define_calendar(cls, name, attendances, tz):
+        return cls.env['resource.calendar'].create({
             'name': name,
             'tz': tz,
             'attendance_ids': [
@@ -20,8 +21,10 @@ class TestResourceCommon(TransactionCase):
                 for index, att in enumerate(attendances)
             ],
         })
-    def _define_calendar_2_weeks(self, name, attendances, tz):
-        return self.env['resource.calendar'].create({
+
+    @classmethod
+    def _define_calendar_2_weeks(cls, name, attendances, tz):
+        return cls.env['resource.calendar'].create({
             'name': name,
             'tz': tz,
             'two_weeks_calendar': True,
@@ -39,42 +42,43 @@ class TestResourceCommon(TransactionCase):
             ],
         })
 
-    def setUp(self):
-        super(TestResourceCommon, self).setUp()
+    @classmethod
+    def setUpClass(cls):
+        super(TestResourceCommon, cls).setUpClass()
 
         # UTC+1 winter, UTC+2 summer
-        self.calendar_jean = self._define_calendar('40 Hours', [(8, 16, i) for i in range(5)], 'Europe/Brussels')
+        cls.calendar_jean = cls._define_calendar('40 Hours', [(8, 16, i) for i in range(5)], 'Europe/Brussels')
         # UTC+6
-        self.calendar_patel = self._define_calendar('38 Hours', sum([((9, 12, i), (13, 17, i)) for i in range(5)], ()), 'Etc/GMT-6')
+        cls.calendar_patel = cls._define_calendar('38 Hours', sum([((9, 12, i), (13, 17, i)) for i in range(5)], ()), 'Etc/GMT-6')
         # UTC-8 winter, UTC-7 summer
-        self.calendar_john = self._define_calendar('8+12 Hours', [(8, 16, 1), (8, 13, 4), (16, 23, 4)], 'America/Los_Angeles')
+        cls.calendar_john = cls._define_calendar('8+12 Hours', [(8, 16, 1), (8, 13, 4), (16, 23, 4)], 'America/Los_Angeles')
         # UTC+1 winter, UTC+2 summer
-        self.calendar_jules = self._define_calendar_2_weeks('Week 1: 30 Hours - Week 2: 16 Hours', [
+        cls.calendar_jules = cls._define_calendar_2_weeks('Week 1: 30 Hours - Week 2: 16 Hours', [
             (0, 0, 0, '0', 'line_section', 0), (8, 16, 0, '0', False, 1), (9, 17, 1, '0', False, 2),
             (0, 0, 0, '1', 'line_section', 10), (8, 16, 0, '1', False, 11), (7, 15, 2, '1', False, 12),
             (8, 16, 3, '1', False, 13), (10, 16, 4, '1', False, 14)], 'Europe/Brussels')
 
-        self.calendar_paul = self._define_calendar('Morning and evening shifts', sum([((2, 7, i), (10, 16, i)) for i in range(5)], ()), 'Brazil/DeNoronha')
+        cls.calendar_paul = cls._define_calendar('Morning and evening shifts', sum([((2, 7, i), (10, 16, i)) for i in range(5)], ()), 'Brazil/DeNoronha')
 
         # Employee is linked to a resource.resource via resource.mixin
-        self.jean = self.env['resource.test'].create({
+        cls.jean = cls.env['resource.test'].create({
             'name': 'Jean',
-            'resource_calendar_id': self.calendar_jean.id,
+            'resource_calendar_id': cls.calendar_jean.id,
         })
-        self.patel = self.env['resource.test'].create({
+        cls.patel = cls.env['resource.test'].create({
             'name': 'Patel',
-            'resource_calendar_id': self.calendar_patel.id,
+            'resource_calendar_id': cls.calendar_patel.id,
         })
-        self.john = self.env['resource.test'].create({
+        cls.john = cls.env['resource.test'].create({
             'name': 'John',
-            'resource_calendar_id': self.calendar_john.id,
+            'resource_calendar_id': cls.calendar_john.id,
         })
-        self.jules = self.env['resource.test'].create({
+        cls.jules = cls.env['resource.test'].create({
             'name': 'Jules',
-            'resource_calendar_id': self.calendar_jules.id,
+            'resource_calendar_id': cls.calendar_jules.id,
         })
 
-        self.paul = self.env['resource.test'].create({
+        cls.paul = cls.env['resource.test'].create({
             'name': 'Paul',
-            'resource_calendar_id': self.calendar_paul.id,
+            'resource_calendar_id': cls.calendar_paul.id,
         })
diff --git a/addons/resource/tests/test_resource.py b/addons/resource/tests/test_resource.py
index f15058c26ce..27127b630e3 100644
--- a/addons/resource/tests/test_resource.py
+++ b/addons/resource/tests/test_resource.py
@@ -6,7 +6,7 @@ from pytz import timezone, utc
 
 from odoo import fields
 from odoo.exceptions import ValidationError
-from odoo.addons.resource.models.resource import Intervals
+from odoo.addons.resource.models.resource import Intervals, sum_intervals
 from odoo.addons.resource.tests.common import TestResourceCommon
 from odoo.tests.common import TransactionCase
 
@@ -1148,3 +1148,72 @@ class TestTimezones(TestResourceCommon):
             (date(2018, 4, 12), 8),
             (date(2018, 4, 13), 8),
         ])
+
+class TestResource(TestResourceCommon):
+
+    def test_calendars_validity_within_period(self):
+        calendars = self.jean.resource_id._get_calendars_validity_within_period(
+            utc.localize(datetime(2021, 7, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 7, 30, 17, 0, 0)),
+        )
+        interval = Intervals([(
+            utc.localize(datetime(2021, 7, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 7, 30, 17, 0, 0)),
+            self.env['resource.calendar.attendance']
+        )])
+
+        self.assertEqual(1, len(calendars), "The dict returned by calendars validity should only have 1 entry")
+        self.assertEqual(1, len(calendars[self.jean.resource_id.id]), "Jean should only have one calendar")
+        jean_entry = calendars[self.jean.resource_id.id]
+        jean_calendar = next(iter(jean_entry))
+        self.assertEqual(self.jean.resource_calendar_id, jean_calendar, "It should be Jean's Calendar")
+        self.assertFalse(jean_entry[jean_calendar] - interval, "Interval should cover all calendar's validity")
+        self.assertFalse(interval - jean_entry[jean_calendar], "Calendar validity should cover all interval")
+
+        calendars = self.env['resource.resource']._get_calendars_validity_within_period(
+            utc.localize(datetime(2021, 7, 1, 8, 0, 0)),
+            utc.localize(datetime(2021, 7, 30, 17, 0, 0)),
+        )
+        self.assertEqual(1, len(calendars), "The dict returned by calendars validity should only have 1 entry")
+        self.assertEqual(1, len(calendars[False]), "False (default) should only have one calendar")
+        false_entry = calendars[False]
+        false_calendar = next(iter(false_entry))
+        self.assertEqual(self.env.company.resource_calendar_id, false_calendar, "It should be company calendar Calendar")
+        self.assertFalse(false_entry[false_calendar] - interval, "Interval should cover all calendar's validity")
+        self.assertFalse(interval - false_entry[false_calendar], "Calendar validity should cover all interval")
+
+    def test_performance(self):
+        calendars = [self.calendar_jean, self.calendar_john, self.calendar_jules, self.calendar_patel]
+        calendars_len = len(calendars)
+        self.resources_test = self.env['resource.test'].create([{
+            'name': 'resource ' + str(i),
+            'resource_calendar_id': calendars[i % calendars_len].id,
+        } for i in range(0, 50)])
+
+        start = utc.localize(datetime(2021, 7, 7, 12, 0, 0))
+        end = utc.localize(datetime(2021, 7, 16, 23, 59, 59))
+        with self.assertQueryCount(12):
+            work_intervals, _ = self.resources_test.resource_id._get_valid_work_intervals(start, end)
+
+        self.assertEqual(len(work_intervals), 50)
+
+    def test_get_valid_work_intervals(self):
+        start = utc.localize(datetime(2021, 7, 7, 12, 0, 0))
+        end = utc.localize(datetime(2021, 7, 16, 23, 59, 59))
+        work_intervals, _ = self.jean.resource_id._get_valid_work_intervals(start, end)
+        sum_work_intervals = sum_intervals(work_intervals[self.jean.resource_id.id])
+        self.assertEqual(58, sum_work_intervals, "Sum of the work intervals for the resource jean should be 40h+18h = 58h")
+
+    def test_get_valid_work_intervals_calendars_only(self):
+        calendars = [self.calendar_jean, self.calendar_john, self.calendar_jules, self.calendar_patel]
+        start = utc.localize(datetime(2021, 7, 7, 12, 0, 0))
+        end = utc.localize(datetime(2021, 7, 16, 23, 59, 59))
+        _, calendars_intervals = self.env['resource.resource']._get_valid_work_intervals(start, end, calendars)
+        sum_work_intervals_jean = sum_intervals(calendars_intervals[self.calendar_jean.id])
+        self.assertEqual(58, sum_work_intervals_jean, "Sum of the work intervals for the calendar of jean should be 40h+18h = 58h")
+        sum_work_intervals_john = sum_intervals(calendars_intervals[self.calendar_john.id])
+        self.assertEqual(26 - 1 / 3600, sum_work_intervals_john, "Sum of the work intervals for the calendar of john should be 20h+6h-1s = 25h59m59s")
+        sum_work_intervals_jules = sum_intervals(calendars_intervals[self.calendar_jules.id])
+        self.assertEqual(31, sum_work_intervals_jules, "Sum of the work intervals for the calendar of jules should be Wodd:15h+Wpair:16h = 31h")
+        sum_work_intervals_patel = sum_intervals(calendars_intervals[self.calendar_patel.id])
+        self.assertEqual(49, sum_work_intervals_patel, "Sum of the work intervals for the calendar of patel should be 14+35h = 49h")