Problem with date and getting last month

I've got a really weird problem that perhaps is more server related than PHP related, but well, I don't know.

If I do the following in PHP:

date("M")

Code (markup):

I get: Oct
Which is correct.

If I try to get the date from the previous month by using:

date("M",strtotime("-1 month"))

Code (markup):

I get: Oct
Which is not correct as I would like it to say Sep.

date("Y-m-d")
date("Y-m-d",strtotime("-1 month"))
date("Y-m-d", mktime(0, 0, 0, date("m")-1, date("d"), date("Y")))

Code (markup):

Gives me:
2011-10-31
2011-10-01
2011-10-01

Can anyone figure out what's going on?

(Date and hwclock on my server gives the same time as date()

 

strtotime("-1 month")
it's decreasing 30 days in spite of 31 days which is creating the problem.

 

But why does it do that? date("m")-1 does the same, which I find strange?