it's been awhile, but I noticed it's available on my gmail for android. after composing a message, tap the three dot overflow menu NEXT TO the paper airplane send icon.
useful to send yourself a reminder - and I like the fact that (unlike snooze) it doesn't remove the original from your mailbox. It's also a useful / respectful feature imo when emailing co-workers outside of normal working hours, if you are not wanting a reply until normal working time (although I use outlook for this)