Ah, I was wondering.
I noticed some problems when trying to use "month" years ago, and had some hope it had been worked out by now, apparently not
It seems that what I consider to be a month varies, i.e. 30 days, 31 days, 28/29 for February, from what PHP considers to be a "month", i.e. "30 days" regardless.
That is, "month" seems to work OK most times, except for "beginnings/endings" if you know what I mean.
Perhaps it would be better to use something other than "month"?