Personally, I would worry about stressing them out, and causing early hermaphrodites. If you only pull a tarp for two weeks, the natural time difference in the two days is almost nothing. So I wouldn't know when you would pull in the first place, 5 min early, 10 min early, an hour early? Then, when they begin flowering, do you wait until the daylight hours match your new light schedule, or do you just stop pulling Aug 1st? All good questions.
Since I worry the most about hermi's, if I decided to pull a tarp like your saying, I might be inclined to continue to pull the tarp at the same time until the end of the season, or at the minimum wait until it's obviously dark at the time you pull your tarp to ensure no stress. However this is my first dep season, and I am no pro. Maybe somebody else can chime in.