That's the "cubic date"! It's just a cubic equation with roots that form the date (in dd/mm/yy form) when subject to the inequalities. It's an overly complicated and gimmicky format, but it's only for fun - so it's unimportant. The format and inequalities likely give away that the roots are equal to the date.
To find the cubic date format for any (valid) day you'd like, you can enter it down into the date input below!
Please insert date or select payment type!