monthDayCountSafe static method

@useResult int monthDayCountSafe({ required int? year, required int month, })

Returns the number of days in month, tolerating a null year and an out-of-range month without throwing.

This is the null-year-tolerant, non-throwing companion to monthDayCount. Two contract differences make it safe for partial or untrusted date parts:

• Nullable year: when year is null, February returns 28 because a leap year cannot be resolved without a year. A known leap year still yields 29. (monthDayCount requires a non-null year.)
• No throw on bad month: any month that is not February and not in the 31-day set returns 30, including out-of-range values such as 0, 13, -1, or very large ints. (monthDayCount throws ArgumentError for month outside 1–12.)

The silent 30 for an invalid month is intentional but is a footgun: validate month separately (e.g. via isValidDateParts) when you need to reject bad input rather than coerce it. Negative years are accepted and follow the proleptic Gregorian leap rule (e.g. -4 is a leap year).

Example:

DateTimeUtils.monthDayCountSafe(year: 2024, month: 2); // 29 (leap)
DateTimeUtils.monthDayCountSafe(year: 2023, month: 2); // 28
DateTimeUtils.monthDayCountSafe(year: null, month: 2); // 28 (year unknown)
DateTimeUtils.monthDayCountSafe(year: null, month: 1); // 31
DateTimeUtils.monthDayCountSafe(year: 2024, month: 13); // 30 (no throw)

Audited: 2026-06-12 11:26 EDT

Implementation

@useResult
static int monthDayCountSafe({required int? year, required int month}) {
  // February is the only month whose length depends on the year, so it is
  // the only branch that consults the leap-year rule — and only when the
  // year is known. An unknown year cannot resolve leap-ness, so it falls
  // back to the always-safe 28 (the minimum February length).
  if (month == 2) {
    if (year != null && isLeapYear(year: year)) {
      return DateConstants.daysInFebLeapYear;
    }

    return DateConstants.minDaysInAnyMonth;
  }

  // Non-February: a membership test (not a table index) keeps the lookup
  // total over all ints, so out-of-range months coerce to 30 instead of
  // throwing — the deliberate no-throw contract.
  return _thirtyOneDayMonths.contains(month)
      ? DateConstants.daysInThirtyOneDayMonth
      : DateConstants.daysInThirtyDayMonth;
}