How do i get the date for (eg) a week ago

Friday 05 March 2004 5:07:46 am

currentdate() returns the current date and time. How can i modify the results, so that e.g I can get the date for a week ago or a month ago.

I want to use the result to compare with date values held in my classes so the results have to be compatible.

I've tried adding and subtracting data and piping it through datetime but it doesnt work. i.e

{currentdate()|datetime(custom,"%Y/%m/%d")} works

but

{currentdate() + <i>any number</i>|datetime(custom,"%Y/%m/%d")} doesnt

Friday 05 March 2004 9:25:32 am

Thanks ! wasnt aware that I shouldnt use + and -