Since the calculation work can be performed by either PHP or the DB, restrict the number of times you touch the DB to a minimum.
Due to that type of consideration, if the calculated date is calculated using dates already in the database, and is used as a filter of some kind for when retrieving info from the database, it can be better to have the database perform the calculation work.
In other words, where practical, use PHP.
If you're already communicating with the DB and the date work can be easily incorporated in to the same request, use the DB.
Try not to request dates from the DB, calculate using PHP, then request info using the calculated dates. That uses two trips to the database. Try to do it with just one trip to the DB instead.