If your final payment is due, say, 60 days before the charter begins, and the charter date changes, shouldn't the payment date change?

Corollary question: is there a distinction between the government closes the border but the charter company is still prepared to do the charter and the charter company closes their base? Compare this to a house rental after Irma. It strikes me there is a difference between the house is useable but you don't want to go and the house blew away.