Right now, when I send regular messages, the tentative time I put in is saved for when I schedule it, unless I leave it blank, then it's immediately.  When I do an A/B test, I put the tentative time in the original message, and when I go to do an A/B test, I always have to remember to change it back to the scheduled time, because the A/B test wipes out the original time/date and makes it "immediately".  It's stressful and I always triple check it before I even start entering in the audiences and then triple check it again before I "Start the A/B test!".