Ha, here's a little more nuanced answer.
I got out to the Newman Lot about 20 minutes before sunrise and the temp was either 21 or 23 degrees, depending on whether you believe Garmin or the Internet — either way, plenty cold to freeze things. The temperatures only dipped below freezing at about 8PM last night, however, which provided enough sub-freezing time to firm-up the drier dirt areas (crunchy), but was insufficient to freeze the existing puddles of water (they just had a thin sheet of ice on top). The areas of rutted dirt around the puddles were soft-ish and yielded somewhat to the weight of the fatbike+rider. I suppose that a narrow-tired bike would have sunk in to a greater degree.
It’s been a while since I’ve done a “daybreak” ride, so I’d forgotten how long it takes to really get a good, hard freeze. Clearly, it’s more than ~12 hours (unless MUCH colder?), and I’m thinking now that you're probably best off with a sub-freezing preceding day.