You don't need to be aware of the time to use an alarm clock. On a phone, you can set it to an appropriate time with a message. When it rings, start doing whatever the message tells you to do.

I'm very absent-minded and frequently forget things like eating and cleaning, but out of respect for other people, I frequently take notes and set alarms, which at least helps me stick to agreed schedules.

Another useful way to manage it is to schedule things loosely when possible. Agree to meet at some point in the span of an hour under circumstances where neither party will be bored waiting. For example, invite the other party over an hour before going to a see a film.