If the facts are as stated, that's crazy. I have the advantage of being older (51), but I always time my credit card bill payments to get there 3-4 days before the due date. (I use electronic bill payment, so less dependence of the Postal Service to get it there on time, and always pay in full.)

As it is, you have only 2 weeks between receipt of the bill and the due date- not the month it used to be. I'm sure it's legal because that's true for all 3 of my cards- but I'm not going to give them a day extra. If a company ever dropped me for that, I'd never open an account with them again.

And Diners Club does have access to the OP's credit records already- many credit card companies have been known to increase someone's interest rate after finding that they were making slow payments on another credit card. They can check your credit bureau records any time they want.