My ISP billing system is having a bad day:
"The due date for your invoice [removed] has been extended to 44732, and your payment will be processed on this new date.
If you are not set up for automatic payment or prefer to pay earlier, please arrange a manual payment before 44732 via one of these methods:"
It is nearly 44732 days since 1st January 1900, someone clearly didn't check these automatic emails too well.