When mass editing fields for which a history is recorded (Supervisor, Hourly Cost, Hourly Payroll, Hours Per Day, Overtime Rule, Timesheet Validation Rule, and Time Off policies), if you:

Specify a Start date only:
The new policy defined for the Supervisor will overwrite all existing records from his Start date onwards.
For Example, John is the existing Supervisor (with no ‘End date’ set) for a group of users, let’s say Greg is assigned to be the team’s Supervisor with the effective date set as Oct 12th. When Greg's policy is added, Web TimeSheet will automatically set End date as Oct 11nth for the existing Supervisor John. 

All past records (for before the new start date) will be retained.

Specify a Start and End date:
The new record will overwrite the portion of any existing records with which it overlaps.
For example, Tom is the existing Supervisor (with no ‘End date’ set) for a group of users. Tom is scheduled to take a 6 month leave of absence, so Alice is assigned to be the team's Supervisor for that period, with a ‘Start Date of May 6’ and an ‘End Date of November 6’. When Alice's policy is added, Web TimeSheet will automatically create a new policy for Tom, with a ‘start date of November 7’.

All past records (for before the new start date) will be retained.

Refer the attached screenshots: