This is probably true for most of the ProCurve line but since I've only performed this on a 3500yl and want make any statements other than "I assume it'll work on most ProCurve switches". Here's the steps to set the switch to get it's time from an NTP server such as us.pool.ntp.org.
If your switch has access to the NTP server, it updated the time immediately for me which you'll see in the top right of your Switch Configuration Menu.
While your here, you probably also want to set the timezone which is an offset in minutes from GMT. For example, EST is -5 hours from GMT so you'd enter -300 and save.