If you always set DateFormat before you use it, without any other changes to it in-between where you set it and where you use it, then you don't have to do the save and revert. And a trap with save-and-revert is that there are often ways for program flow to escape before the revert eg if an exception occurs, or you have a break or a return in-between.
On the other hand, from a defensive programming perspective, it's also a good habit to leave things as you found them, just in case.