It's 1 AM and you're lying in bed. It's been a long day and your brain is still overworked with everything you did and everything you still need to do tomorrow. To avoid forgetting things, you shout at your Google Home or Google Assistant on your phone, "Ok Google, set a reminder to read Android Police more diligently tomorrow at 10 AM," or you know, something more critical than that (HOW DARE YOU?!). Then you stop and think... it's technically "tomorrow" already, because we're past midnight, so when did Google actually set the reminder for? In 9 hours or in 33 hours? But you're already dozing off and you can't be bothered to check, so you fall asleep and think that you'll find the answer in the morning. You wake up and lo-and-behold, Google reminds you to read AP more diligently at 10 AM, which is awesome because you then discover this little priceless piece of information we're sharing with you here.
This - or a version of it, after all I don't need a reminder to read AP - has happened with me quite often already and although I took it for granted for the first few times, I once noticed it and realized how smart this little feature is. Google does indeed understand that when you say "tomorrow" and it's past midnight, to you, this is still night and what would technically count as the previous day, so your tomorrow just means in the morning, not in 24+ hours. But I never knew when the cut-off time is exactly. At which point does "tomorrow" really become the next technical day and not just the morning? Well, now we have our answer: 5 AM. Before that, the reminder fires the same day. After that, the reminder fires the next day.
Although only the Google Home reminder page was updated to add this nugget of info, I'm fairly certain this also works on Google Assistant on phones and for calendar events, not just reminders. I can't verify it though now, because it's still mid-day here.
- Google Support