Finding the Last Day of a Month in PHP

Finding the number of days in a month is useful for reporting, billing periods, scheduling, and date validation. PHP can do this directly with its date APIs, including support for leap years.

A Simple getLastDay Function

The following function accepts a month in YYYY-MM format and returns the number of days in that month:

function getLastDay(string $month): int
{
    $date = new DateTimeImmutable($month . '-01');
    return (int) $date->format('t');
}

How the Code Works

1. Create a date for the first day of the month

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   $date = new DateTimeImmutable($month . '-01');

   For an input such as "2022-06", this creates a date representing June 1, 2022.

2. Read the number of days in the month

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   return (int) $date->format('t');

   The t format character returns the number of days in the represented month, from 28 through 31.

Usage Examples

echo getLastDay("2022-01"); // 31
echo getLastDay("2022-02"); // 28
echo getLastDay("2024-02"); // 29
echo getLastDay("2022-04"); // 30

PHP handles leap years automatically, so February 2024 correctly reports 29 days.

Alternative with date()

If you are already working with Unix timestamps, the t format character also works with date():

$timestamp = strtotime('2024-02-01');
echo date('t', $timestamp); // 29

Conclusion

PHP’s t date format makes it easy to determine the number of days in a month without manually maintaining month-length or leap-year rules. DateTimeImmutable is a clear choice when your application already works with PHP date objects.