formula to calculate "previous month"

I have an input cell in which i enter the current month and would like a formula to calculate the previous month based on my current month input cell.

I've tried the "=current month input cell - 1" but this doesn't work :P

The TODAY & NOW functions are no good here i would think as the current date differs from the actual date often by a couple of months.

If you want 1st of Prior Month

=DATE(YEAR(A1),MONTH(A1)-1,1)

If you want last day of Prior Month

=DATE(YEAR(A1),MONTH(A1),0)

If you want same day in Prior Month

=DATE(YEAR(A1),MONTH(A1)-1,DAY(A1))

but note the last of the above is open to error (eg 31st May would generate 1st May rather than 30th April)

You might also consider use of EDATE - prior to XL2007 this warrants activation of the Analysis ToolPak Add-In on each client using the formula (see: Tools -> AddIns)

That holds true only if the current cell is 1st of month.

In essence Date values in XL are Serial Numbers where 24 hours equates to 1

It follows that if you subtract 1 from an existing date you are subtracting 1 day.
So where current date is 1st of month you end up with last day of prior month which - pending latter calcs - may / may not be an issue.

Using the DATE construct whereby Month is adjusted is slightly more robust though again not infallible - all depends on context
(in this instance I'd say you are better off using the DATE based approach)