What I've learned from working with transactional databases is how to deal with things like this.

You have a time in the evening, now that time is blocked by your friend (they didn't commit nor rollback). You can't process further because of that lock on your time. The lock is unspecific (there is no start nor end of time).

So you become anxious, because you need to route things, but you can't do this because of the undefined commitment.

There are several strategies that can work:

  1. kill the undefined commitment (you can tell them that something came up, so you can be there no longer)
  2. specify the bounds of the lock, so that it won't take everything (ask them again for the time and place, if they won't reply do the 1.) 
  3. Guess it. If it is a drink, then it is after work. Then how much it usually takes me to get there, do it + some padding in case something goes wrong. Basically you do the work on your side. 
  4. Discard all conflicts. (Tell everyone that you have an important meeting, so you will interact with them tomorrow). Now your whole day is waiting for that meeting and nothing else. In some circumstances it is the right way to go. 
  5. Optimistic scheduling. Schedule everything to the best of your ability. If something conflicts, kill or reschedule the less important thing. (I am sorry,  but something came up, so I can't be there today, maybe tomorrow?)
  6. Simply wait. As the day progresses you will get more information to make the  right choice