Testing whether a year is a leap year

This lesson covers testing for a leap year in VB.NET, with the aim of understanding how to write logic combining several conditions.

A leap year follows a slightly awkward rule: divisible by 4 but not by 100, or divisible by 400. Testing only for multiples of 4 gets some years wrong.

The sample code expresses that whole rule in the single expression (year Mod 4 = 0 AndAlso year Mod 100 <> 0) OrElse year Mod 400 = 0. 2024 divides by 4 and not by 100, so it is a leap year; 1900 divides by 100 but not by 400, so it is not.

A common early stumble is testing only divisibility by 4. Exceptions such as 1900 — a multiple of 4 that is nonetheless not a leap year — mean the 100 and 400 conditions have to be checked as well.

In professional work, this logic is essential to accurate day counts when implementing date calculations or calendar features.

📖 Reference code

Module Program
    Function IsLeapYear(year As Integer) As Boolean
        Return (year Mod 4 = 0 AndAlso year Mod 100 <> 0) OrElse year Mod 400 = 0
    End Function

    Sub Main()
        Console.WriteLine(IsLeapYear(2024))
        Console.WriteLine(IsLeapYear(2023))
        Console.WriteLine(IsLeapYear(1900))
    End Sub
End Module

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