diff --git a/addons/hr_holidays/models/hr_employee.py b/addons/hr_holidays/models/hr_employee.py
index ff236d8749e..e58d534ed5d 100644
--- a/addons/hr_holidays/models/hr_employee.py
+++ b/addons/hr_holidays/models/hr_employee.py
@@ -308,7 +308,7 @@ class HrEmployee(models.Model):
     @api.model
     def get_public_holidays_data(self, date_start, date_end):
         self = self._get_contextual_employee()
-        employee_tz = pytz.timezone(self._get_tz() if self else self.env.user.tz)
+        employee_tz = pytz.timezone(self._get_tz() if self else self.env.user.tz or 'utc')
         public_holidays = self._get_public_holidays(date_start, date_end).sorted('date_from')
         return list(map(lambda bh: {
             'id': -bh.id,
diff --git a/addons/hr_holidays/models/hr_leave.py b/addons/hr_holidays/models/hr_leave.py
index a62173d7dd8..99bb1ccd7e7 100644
--- a/addons/hr_holidays/models/hr_leave.py
+++ b/addons/hr_holidays/models/hr_leave.py
@@ -1565,7 +1565,7 @@ class HolidaysRequest(models.Model):
 
     def _to_utc(self, date, hour, resource):
         hour = float_to_time(float(hour))
-        holiday_tz = timezone(resource.tz or self.env.user.tz)
+        holiday_tz = timezone(resource.tz or self.env.user.tz or 'UTC')
         return holiday_tz.localize(datetime.combine(date, hour)).astimezone(UTC).replace(tzinfo=None)
 
     def _get_attendances(self, request_date_from, request_date_to):