If it's just dates, rather than time of day, then anchor off of noon rather than midnight with your formulae. Your calculations will potentially be short by one day during spans around Daylight Savings Time transitions if you are adding "days as seconds", 60*60*24*$days -- the DST transition happens at midnight.

Matt

Update: As AM said, 2am. Another option is to do your calculations in gmtime and convert to localtime when your finished.