Changing the next billing date/time for a subscription is a handy way to bill a subscription sooner or later than it would normally happen. Traditionally, you weren't allowed to set a billing time in the past because, well, that's not the future.
We've made some improvements here to give you a little leeway on this in order to ease integrations. Now, you can force a processing by setting the next billing at to "now" or "just before now", and as long as the subscription hasn't renewed and moved the billing period dates while you weren't looking, we'll allow it.
Also, we've added a handy timezone selector to the UI for selecting your next billing date, so you don't have to do any weird timezone math in your head when trying to bill that customer right at 1pm PST tomorrow.