Extend a contract at any time?

[windycity00]

The Lakers extended Bynum's contract this season, but he was going to be a restricted free agent this summer.
So I wondered, can a team/player simply agree to a contract extension at any given time?
Seems like a way to avoid having your star player become a free agent..

[Three34]

Some contracts yes, some no. Most contracts can't be extended at all, some can at any time, some only between certain times. The full explanation is here.

http://members.cox.net/lmcoon/salarycap.htm#Q51