If you have both Scheduling & Timesheet modules enabled, in Settings > Timesheets you can enable feature to automatically pre-fill hours from Schedule:

Timesheet hours are automatically updated from Schedule automatically until that particular day is edited (hours changed or time clocked in/out). 

After that, changes from Schedule will not be sent to that day in timesheets. Basically - once a day is edited in Timesheets, it is "locked" for any further updates from Scheduling.

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