Bi-weekly pay period

I'm writing a quick little thing to act as a time clock since people are writing out by hand and it's not so accurate. It's basically click a button to clock in and click a button to clock out. What I also want to do is create a report (well I've already created the report) but I want to limit it to the current pay period. Our pay periods are biweekly. I was trying to think of a smart way to have php figure out what the current pay period is and I'm having a hard time figuring out if this is even possible or should I just tell it a years worth of pay period ranges for now. I figure something like, if the week # is even, use this past Sunday as the start date. If the week # is odd use the Sunday of the week before. Then figure out what date is 2 saturdays after whatever Sunday was selected. Any assistance appreciated. Sarah