From my understanding and talking with our HR department, SDI waiting period starts the day your doctor says you're unable to work any longer.. There's no specific date or guideline. Most women don't want it to start at 36 weeks, because there's a limit to the total amount of time you can be out (I think 12 weeks?) and most people would rather spend as much of those 12 weeks with their baby as possible.
I'm due 2/16, and planning a tentative out date of 2/1, just for logistical reasons, but assuming I'm still feeling well, will keep working as long as possible