Dates showing up wrong on 31st of the month.

There was a thing in php where strtotime gives the wrong answer at the end or beginning of months for +1 month and -1 month etc. It was, if I remember correctly, something like on April 30th the first day of the previous month was given as Feb 1, not April 1, because the + or - month functionality had a flaw of some sort in it.

However, I can't remember the exact details as I thought it was fixed in php 5.3+ but your problem really reminds me of that issue. The datetime class is much better, easier and more powerful now anyway. So I would suggest not using strtotime but the datetime class http://php.net/manual/en/class.datetime.php to work out your required months.

Can you show how you are calculating -1 months in php? Might be more revealing.