I would like to be able to put a date in Cell A1 (say April 18, 2005)

Then, after recalculating, in cells B1:B24 would be:
03/01/05
02/01/05
01/01/05
12/01/04
etc
- in other words, the first of the month going back 24 months.

Can someone give me a formula for this?

Thanks!
Daniel