When I go to sleep, I usually end up thinking about how long I'm going to be out (usually not enough) so I'd just set the timer for 6 hours from now rather than for 6:00 AM. That's the way my brain works anyway. The best feature on my phone's alarm clock is that it says how far in the future the alarm will go of once it's set. That's also saved me a couple of times when I accidentally set the alarm for the next day.

During the day, it's not too hard to do the math (if I need to know something at 3:00 and it's 11:30, for example).

I haven't tested this yet, but I also imagine myself just setting it two hours ahead, for a time I'd likely need the reminder, but am pretty sure I'd have completely forgotten. As an example, my first message was set for 5 minutes in the future. Then I realized I was late for a meeting and got a call. I had completely forgotten that I had set the set message until I answered.

Also, I noticed that the message started to quickly that I missed the first part as I answered. A second pause or possibly a bumper "This is a message you left on WakeMyself" would help avoid that.