Well, the math seems right but here is the thing. Your cycle is going through an irregular phase which means simple math won't exactly work to calculate it. You can use the app here at MedHelp called MyCycles which can be found at the top of the page under Health Tools. It's very helpful but will just be a best guess of when to expect it. Women often will go through periods of irregularity with cycles of days different than they normally have (fewer or more common, more). Holidays seem to be a notorious time. These things make a longer cycle-- stress, changes in routine, illness, weight gain or loss, etc. Sounds like holiday time, right? Anyway, if you are sexually active, you can always test to be sure but I would guess its on its way and this is perfectly normal. good luck