MB here. You can use your time for whatever you choose OP. It's not my business, but I would have no problem w/ a request like this. I'd enjoy hearing about the concert and might be a little envious.
Our nanny got tix to see Adele a couple of years ago and it was a scenario just like this one - except we also worked it out so she could come in late the next morning since it was such a late night.
You're allowed to have a life!!