Question

I see there is strange problem in php with month addition and subtraction.

My questions are:

does 1 month have an equivalent in days ?

if yes, is this a common standard in all programing languages ?

A few examples:

echo date('Y-m-d',strtotime('2011-03-31 -1 months')); //2011-03-03
echo date('Y-m-d',strtotime('2011-03-30 -1 months')); //2011-03-02
echo date('Y-m-d',strtotime('2011-03-29 -1 months')); //2011-03-01
echo date('Y-m-d',strtotime('2011-03-28 -1 months')); //2011-02-28

Answer 1

From your examples, it looks like it’s subtracting 1 from the month part, and then correcting for illegal dates. Your second example:

2011-03-30 - 1 month = 2011-02-30. This date does not exist, as February 2011 had only 28 days. 30 - 28 = 2, so it puts it as the 2nd day of the following month.

However, I have not found documentation about this.

Either way, assuming I’m right, the answer to your question is no, “1 month” does not have a (constant) equivalent in days, it depends on the input.