If a time entry is created using start/stop times, the hours field should match those times and not be editable. This solves the problem of incorrect time entries.
If someone changes the hours field and that total doesn't match what is entered in the start/stop fields, TS should not let the time entry be saved. It should give a prompt that the fields don't match and require the start/stop time fields to be edited. The hours field should not be editable.
How often would you use this request?
With every time entry.