Microsoft makes available a 120-day time-locked version of Windows NT, and their Beta versions expire 240 days after they are installed. However, Microsoft does not provide any means of determining if the version of NT you are running is a time-locked version or not. Many organizations run into problems when an installation they had forgotten was a trial version suddenly announces it is shutting down. This applet will tell you whether your systems are running trial versions of NT, and if they are, their exact expiration times.
I know of a major web site that went off the air for a time because it had been installed on trial versions of NT server which began to reboot once each hour.
If you get caught in this situation, with a production server running under an evaluation copy, you can try the upgrade procedures supported by Microsoft. Before attempting this, check out U-Deploy which is a commercial utility designed for this purpose. Be sure to check out their cautions about Microsoft's upgrade recommendations which have left people with no option but total reinstall.