Description:
------------
When I run date('m', '-1 month'), I expect to get the number of the previous month, but it is not what always happen.

Today is 2019-10-31, and when I run it I got '10'.

To understand better what is happening, I tried to run date('Y-m-d', '-1 month') and I got '2019-10-01'.

I believe it is because there is no day 2019-09-31, so it rounds up. But IMO it should round down in this case.

Test script:
---------------
$lastMonth = date('m', '-1 month');

echo $lastMonth;

Expected result:
----------------
'9'

Actual result:
--------------
'10'