The Default hours per day value will determine how many hours make a day in Web TimeSheet.

You can modify this option at 2 levels. User level or System Wide.

System Wide setting: When you modify the 'Default hours per day', under System Preferences. (Path: Administration -> System), it dynamically updates all the users historical time off data. This is a global change.

User level:
If you would like only a group of users time off per day to be modified, then you can change the Default hours per day from only those users profile in Web TimeSheet. (Edit the user profile, Under Advanced tab -> Other -> 'Hours Per Day' -> Edit Historical Hours Per Day)

Example: When you edit 'Default hours per day', set a start date in the past and enter a new value for 'Hours Per Day', then the Web TimeSheet will dynamically update the users past time off entries with the new value set.

Note:
You will require Web TimeSheet administrative rights to modify the Default hrs per day value.