Feeling very confused

I'm perplexed. I haven't been this confused in quite some time, and I have to say that this is not a good state to be in. 

So, here's the situation. I just got a new job offer (yay!) a few days ago and according to my phone conversation with the HR representative, the on board date we agreed on is July 1st. But, about two hours ago, I opened the offer letter they mailed me and the on board date on it is June 21st (Monday). 

I'm not surprised to see this mix-up because we went back and forth discussing the date during the phone call, but the dilemma I have now is that today is a Sunday and the 21st is tomorrow, meaning that I have no one to iron out this confusion with before one of the potential dates arrive.

I'm just puzzled...