I just tried to subtract 6 and 5 months respectively from current date 08/29/2015 @ 11:19am (UTC)
and got the same result
Here is the code sample:
date("M, Y", strtotime("-5 months")) // returns Mar, 2015
date("M, Y", strtotime("-6 months")) // returns Mar, 2015
Is it due to day light saving? I think No.