Tax day is the 18th this year, and Sanders has specifically agreed to a debate on Good Morning America in the past. Stop making excuses for him.
A debate when the other person is already scheduled to make an appearance at a public event, television, etc is a bit silly. If Clinton was holding some event, would be on a television show on a certain date and Sanders offered a debate then instead it would be just as silly.
The only one that seems remotely like a good faith effort is the one on the 14th, but of course without knowing details it is hard to say. With that being said I would like to know the reasons behind Sanders campaign denying the 14th (if that is indeed true), as well as their proposed times that they have indicated Clinton has said no to. And of course the original date mentioned by Clinton, if anything shows it wasn't a realistic proposal.