This thread describes many potential culprits.
Check your settings under "display" and make sure it's set to the time you want it to be.
Another cause might be that an app could be wakelocking your phone so the screen stays on. There's an app for that. (To detect it, that is).
Also if you have Pandora installed, look in the Pandora settings and make sure it's set to conserve battery. The Wakelock app (linked above) would probably detect it, but I figure Pandora's common enough that it's worth mentioning directly here just in case.