What?

"You've already watered your plant today, please return tomorrow to water it again. You can water again in 0 hours."

(I planted this hours ago...)

Also, when you plant something, is it supposed to be 24 hours from that time? Or just the next day, after midnight Subeta time?

When it says 0 hours like that, it's usually because the time remaining until you can water again is less than one hour, but the timer only counts in hours so it won't say like 45 minutes or 23 minutes or whatever. So it just says 0 hours because it's not enough time to make a full hour, but you still have some time left to wait.

It doesn't matter if you water it on the hour exactly. It just has to be sometime within the next bunch of 24 hours (in relation to you). This means sometimes you will only water your plant twice and it will come out okay.