changing working times on an existing Project file

Try this to force the new calendar to apply:

1.    Update the Project Calendar
o    Go to Project → Project Information → Calendar and select your new calendar.
2.    Check Resource Calendars
o    Open the Resource Sheet.
o    For each resource, ensure their calendar is based on your new calendar (or update their working times to match).
3.    Recalculate task schedules
o    After changing calendars, force Project to recalc: 
    Select all tasks (Ctrl+A in Gantt view).
    Use Task → Information → Advanced tab → Calendar and clear any custom task calendars.
    Then press F9 (Calculate Project).
o    This will reapply the new calendar’s working times.
4.    If tasks still don’t shift
o    Check if constraints (e.g., “Must Start On”) are locking them. Remove or loosen constraints.
o    Verify “Scheduling ignores resource calendars” is not blocking changes.