BoxOfRox:
I'm not familiar with Navient, but I do know that FedLoan Servicing's dates are often off.
The best thing to do is to keep a spreadsheet and make sure it matches up with what they are saying your balance, interest calculations, etc. are supposed to be. My interest is rarely correct.
Another suggestion if you don't get it in writing in a timely matter is to request it in writing, either by snail mail or fax. Don't provide a phone number. They will have to return the requested information in print.