Dec + 9 months = Aug.
Conception occurs 9 months prior to birth.
Get it?
Although by my reckoning, it should be Nov...
Jan 1 (1/1) + 9 months -> Oct 1 (10/1).
Dec 1 -> Sep 1.
Nov 1 -> Aug 1.
S'okay torifile, fencepost errors trip up a lot of non-programmers... and way too many programmers.