Problems with time (being unaware of it's passage, mixing up/forgetting appointment times, having difficulty projecting it, ie "I need to leave at 3:15 to reach my destination at 4:45) are pretty common after concussion.

You are in very early days, so try not to fret too much. In the interim, you might want to try to remember to write appointment times out in a day planner. Reminder aps on phones and tablets can be helpful, too.

If you do make a mistake, don't beat yourself up over it.