If i was you i would google payroll system and do some research what others have done.
In the end you will probably need way more than employees and hours.
To answer the specific question, you can create a calculation field that uses the date on the time card record to calculate a date for the first day of the same week. The following assumes Sunday as the first day of the week:
Date - DayOfWeek ( Date ) + 1
You can then sort by this field to group the records by week in a summary report or use a relationship or filtered portal to show only those records for specific week.