Regular Disney tickets are always "Active," meaning you don't have to visit a ticket booth and show ID, etc., in order to use them. When you go thru a theme park "Mickey Head" turnstile for the first time, that begins your 14-day expiration clock. If you visit a park on the 12th, your last date to use your ticket would be the 25th. If you started using your tickets on the 13th, your tickets would be usable thru the end of the 26th.
To figure last day of a 14-day expiration, pinpoint your start date on any monthly calendar. For example, let's choose Thursday, March 12th. Go down the calendar vertically two weeks, to Thursday March 26th, then back-up one day. Wednesday, March 25th would be the last day you could use your tickets.