It's usually done in bulk, so the overall payoff is the combination of value and number of targets, but the effort is typically sublinear with the targets. Something easier to attack but relatively low in number is not as juicy as something a bit harder (where the effort is mostly a one-off up-front rather than per target) but having many, many more targets.
It's because units up to hours are of a fixed size, but days in most places are only 24h for ~363/365 days of the year, with some being 23h and some being 25h.
(This is ignoring leap seconds, since the trend is to smear those rather than surface them to userspace.)
The map iteration order was always "random", but imperfectly so prior to Go 1.3. From memory, it picked a random initial bucket, but then always iterated over that bucket in order, so small maps (e.g. only a handful of elements) actually got deterministic iteration order. We fixed that in Go 1.3, but it broke a huge number of tests across Google that had inadvertently depended on that quirk; I spent quite a few weeks fixing tests before we could roll out Go 1.3 inside Google. I imagine there was quite a few broken tests on the outside too, but the benefit was deemed big enough to tolerate that.