This phrase derives from a concept found in Jewish texts where למספרם refers to the method of counting dates "according to their (non-Jewish) reckoning."

• החשבון (hacheshbon) means "the calculation" or "reckoning."
• למספרם (le'misparam) specifically refers to "their count," often used to indicate the non-Jewish (Gregorian) way of counting time.

In this context, החשבון למספרם would mean the calculation or date based on the secular calendar, as opposed to the Jewish method of dating according to the Hebrew calendar.