Calendar Anomalies — The Leap Year Trap

The Gregorian calendar is not a clean mathematical system. It is a historical compromise between astronomical reality and political expediency, accumulated over two millennia. The result is a calendar with months of four different lengths, a year that is sometimes 365 days and sometimes 366, and a set of rules for determining which is which that trips up even experienced developers.

This module examines the structural irregularities of the Gregorian calendar and their consequences for software testing. You will learn why February behaves differently from every other month, how the leap year algorithm works and where implementations commonly get it wrong, what happens when date arithmetic crosses month and year boundaries, and how to design test cases that systematically expose these calendar edge cases.

The four lessons in this module progress from history to algorithm to boundary cases to arithmetic. By the end, you will have a complete mental model of calendar irregularities and a practical toolkit for testing date fields that depend on them.