If you set the time from 0:00 to 0:00 it will still upgrade.
Note:- If the clients in the group run a version of Symantec Endpoint Protection previous to MR3, turn off scheduling. Scheduling is on by default when a new client install package is added to a group. If scheduling is turned on, the upgrade fails. To turn off scheduling, in the Add Client Install Package dialog box, uncheck Upgrade Schedule.