If you press the pre-med timer at a later time than the pre-med was actually given (ie pre-med given at 10:15, you remembered to press "Start" on anesthetic sheet at 10:30), and need to change the time to 10:15, you can do so after the anesthetic sheet is created.  You just need to erase your initials from the 10:30 cell for that medication and add them to the 10:15 cell instead.

The pre-med start time does not start an actual timer, only the induction medication does.