Is it possible to edit specific hour availability?

My availability for certain days are different - I only have a few hours availability on Tuesday, but all day Monday for example. Why is it not possible to edit for specific days? Also, my job is random and partially on call, so my availability changes by the day and week. It is supremely frustrating and frankly, quite bizarre that there doesn't seem to be a way to do this. It means that my availability will show as 11am-3pm every weekday, so clients will try to book for that time, but if it's on a Tuesday for example I'll have to turn them down which would make them think I'm unreliable and less likely want to book in future. Is there any explanation why this basic feature isn't available?