**problem:**
On timesheets, the overtime indication next to an employee's name is incorrect when using flexible work schedules. for example: a "Flexible 20h" schedule (4h a day) shows 1h of negative overtime even when the employee has logged exactly 20h for the week.

**steps to reproduce:**
1. Create a new working schedule with flexible hours enabled for example (20h/week, 4h/day average)
2. Assign this schedule to an employee
3. Go to Timesheets, search for the employee
4. Navigate to a past week
5. Enter 4h on each working day
6. Observe the overtime indication shows incorrect value (-01:00)

**cause:**
In `resource/models/resource_calendar.py`, the flexible hours algorithm that determines the date range by converts UTC boundaries to the employee's timezone. When the employee's timezone has a positive UTC offset (UTC+1, like in brussels time zone), `Sun 23:59:59 UTC` becomes `Mon 00:59:59 CET`, pushing `end_date` to the next Monday. This creates an 8 day range instead of 7. The algorithm then starts a new weekly budget for the spillover day and allocates 1 extra hour, making `allocated_hours` 20.9999998 instead of 20.

**fix:**
- Use the UTC date before conversion to the employee's timezone when determining the flexible date range.
- prefer `self` when it is the flexible calendar being queried, so hr_contract's `_get_calendar_at()` override cannot substitute the contract's calendar parameters (full_time_required_hours, hours_per_day) for the flexible ones.

**note**
Updating the test (`test_no_carried_over_leaves_for_flexible_resource`) in `hr_holidays/tests/test_expiring_leaves.py` expected duration logic, is to match the corrected inclusive day range and prevent asserting the previous spillover behavior.
link to the enterprise PR: https://github.com/odoo/enterprise/pull/112879

link to the community PR: https://github.com/odoo/odoo/pull/257269

opw-5970511

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Forward-Port-Of: odoo/enterprise#115435
Forward-Port-Of: odoo/enterprise#112879