Changing month jumps two months

[IanRCarter]

Firstly, this isn't an issue today, the first of the month. I did however notice it yesterday but didn't get chance to report it.

So yesterday was 31st January. When I clicked forward one month, I'd expect it to go to February, however, it skipped to March. Clicking previous from here took me to February.

I haven't looked at the source code but I'm guessing it's possibly trying to advance 1 month from now to 30th/31st February which obviously doesn't exist so it goes forward another month.
If the cause of the issue is along those lines, I'd assume you could fix it by first setting the current date to the 1st and then adding 1 month?