most people use this phrase to emphasize the important date.
e.g. you should submit your homework on or before Dec 31, 2007 otherwise you will be punished.
2007-12-24 11:30:24 補充:
在這時間或之後you could use the opposite word of BEOFRE - AFTER, i.e. on or afteryou should open the present on or after Dec 25, 2007.