Create file name using date and time

I hope you could help me, I'm trying to call in the date from another class and looks like "2011-03-09 06-57-40", I want to use this to create the file below but everytime I do when the output runs it creates a new file as it re-runs calling the dat(). I know what's going wrong I'm just not sure how to fix it, I want to permently writw to the same file. I hope this makes sense? :/I hope you could help me, I'm trying to call in