Tried before 1/2 years ago and had same issue, won't let me submit future time. So I switched to Chrome, their Snooze works way better. Not only does it let you input future date(with calendar) but it also lets you set Repeats, which is very helpful. I prefer Firefox, but will try another Snooze as these issues have never been fixed, on this Add on.

I like the idea of this add-on a lot, but when snoozed tabs woke up, they often either woke up repeatedly (even though I only snoozed them once), or many duplicates of the same tab would wake up at once (sometimes close to 10 tabs with the same snoozed webpage).