diff --git a/addons/resource/models/resource.py b/addons/resource/models/resource.py
index 9c815aacb1a..d21a7556801 100644
--- a/addons/resource/models/resource.py
+++ b/addons/resource/models/resource.py
@@ -629,12 +629,14 @@ class ResourceCalendar(models.Model):
         def interval_dt(interval):
             return interval[1 if match_end else 0]
 
+        tz = resource.tz if resource else self.tz
         if resource is None:
             resource = self.env['resource.resource']
 
         if not dt.tzinfo or search_range and not (search_range[0].tzinfo and search_range[1].tzinfo):
             raise ValueError('Provided datetimes needs to be timezoned')
-        dt = dt.astimezone(timezone(self.tz))
+
+        dt = dt.astimezone(timezone(tz))
 
         if not search_range:
             range_start = dt + relativedelta(hour=0, minute=0, second=0)
diff --git a/addons/resource/tests/test_resource.py b/addons/resource/tests/test_resource.py
index 27127b630e3..25ae1b80ee8 100644
--- a/addons/resource/tests/test_resource.py
+++ b/addons/resource/tests/test_resource.py
@@ -533,6 +533,21 @@ class TestResMixin(TestResourceCommon):
             datetime_tz(2020, 4, 2, 18, 0, tzinfo='UTC')
         ), "It should have found the start and end of the shift on the same day on April 2nd, 2020")
 
+    def test_adjust_calendar_timezone_before(self):
+        # Calendar:
+        # Every day 8-16
+        self.jean.tz = 'Japan'
+        self.calendar_jean.tz = 'Europe/Brussels'
+
+        result = self.jean._adjust_to_calendar(
+            datetime_tz(2020, 4, 1, 0, 0, 0, tzinfo='Japan'),
+            datetime_tz(2020, 4, 1, 23, 59, 59, tzinfo='Japan'),
+        )
+        self.assertEqual(result[self.jean], (
+            datetime_tz(2020, 4, 1, 8, 0, 0, tzinfo='Japan'),
+            datetime_tz(2020, 4, 1, 16, 0, 0, tzinfo='Japan'),
+        ), "It should have found a starting time the 1st")
+
     def test_adjust_calendar_timezone_after(self):
         # Calendar:
         # Tuesdays 8-16