Token to display dynamic date

I'm trying to create a custom token that would act as a starting date. The starting date will change every 2 weeks (it is the start of a payroll period).

For example, the start date should first be 2014-12-22, then 2015-01-05, then 2015-01-19, and so on for every two weeks. My goal is to use the "startdate" token to populate a timesheet webform to be filled out. I believe I could use "startdate + 1 day" for the next day, "startdate + 2 days" for the 2nd day following the start date, and so on.

Basically, I need to create a "startdate" token that uses a certain date when "today" falls between two dates.

Any tips or advice?

Thanks