I'm fortunate to be in a field where there is some flexibility. I teach at a community college, so I can pretty much set my office hours so that I can leave for some early games if needed. My only restriction is that I have to have at least 30 hours on campus, so that gives me some flexibility too, as long as I teach my classes.

What I've done is give my assignor my availability by stating what time my last class ends for the day and let the assignor figure out where I can be and at what time. I've had no problems so far, and matter of fact, this year I'm noticing some of my games are getting to be a little later, which means more time in my office before I head to my games. Nice.

As for needing to leave early (which happens more on Fridays during football season because we travel to some far away games), my program chair has been understanding. I think it helps I'm here from 7 AM to 9 PM on Mondays during the fall semester (night class). My dean has been great about it, as long as I teach my classes and put in 30 hours a week. She knows I'm here at least 40 because she's seen my office lights on before 7 in the morning sometimes.

Important thing is, be upfront with your supervisor about it, and it doesn't hurt to let him/her know what kind of a schedule you have. Give some kind of advance notice (ie, I'm giving my dean and program chair my basketball schedule now that the subvarsity games have been finalized). If the supervisor isn't understanding, then you would have to make a tough decision. But, the more open and cordial you are about your schedule and obligations outside of work, the more open and cordial your supervisor may be.