It actually works fairly simply. In the old version you just set the start time in race creation and as soon as you opened up the clock it worked. Now you open up the screen then edit the start time from the timing screen. Two things to be aware:
1. I always have to do it twice ... the first time the seconds are off.
2. The time on the timing screen will be off by an hour. The time on the Public display will be correct. I have reported this several times. It is disconcerting but it works every time.
Remember, you actually start the race then edit the clock then click the box that says Actual Start Date and Time.