Originally Posted by jbeckett

A "calendar year" starts on January 1 and ends on December 31. I also believe that Hilton provides a grace period to March 31 of the following year in which you keep your status.

All of this assumes that you earned your status by staying 30 times, or 60 nights, or earning 120000 base points.
If you obtained status through some other means (credit card, status match or challenge, status gift), then the expiration may be different. For example, if you have Diamond status via the Ascend credit card, I believe that you will lose your status immediately if you cancel the credit card.