Saturday morning I started my D140 [same engine as the OP] with a going on two-year old 230 CCA battery having a date code of 03/19. Overnight temperature had dropped to 28F, and I imagine that the battery wasn't much warmer than that when I started the mower. In any event, the first half second of cranking sounded weak, but then she fired right up. She hadn't been driven in two weeks either.
The original battery was only 195 CCA. It served me well for four years but gave up the ghost at the beginning of season five. I don't recall ever attempting to start the mower at such a low temperature, so the initial half second observation may be completely meaningless.
Thumbs up to you guys with the AGM batteries; that's the way to go if you can afford them and have a long term outlook.