As with most loyalty rewards programs, airline elite status benefits have arcane and complex rules about how they can be applied. For most U.S. carriers, the deadlines for accruing rewards points to clinch elite status coincide with the calendar year: you have from Jan. 1st to Dec. 31st to earn points. Confusingly, once you earn elite status, it stays good for the rest of the calendar year you earned it in and then remains valid for around the next thirteen months. This is because you get “complementary” elite status for the first calendar year and the deadlines for elite status usually run from Jan. 31st to Feb. 1st of the next year.
Read the full article here: When Does My Airline Status Expire?