First and last day of month

Use it free in the browser for billing windows, report ranges, and homework, or call the API at $0.002 per successful request when you need reproducible month bounds in CI, scheduling pipelines, or product features that must not invent end-of-month dates by hand.

What month start and end mean and what the response contains

Knowing the first and last calendar day of a month answers a different question from counting days between two arbitrary dates, finding the next weekday, or converting Julian day numbers: it fixes the civil bounds of a named year-month so that report ranges, invoices, and retention windows open on day one and close on the true last day without off-by-one errors at thirty-day months or leap Februaries. If you need whole-day distance between two free dates, weekday navigation, or ISO week conversion, use those dedicated tools; this page answers only the calendar start and end of a single validated year-month. You supply an integer year and an integer month from one for January through twelve for December; the calculator validates those fields, looks up how many days that month has in that year under the proleptic Gregorian calendar, and returns first_date as YYYY-MM-01, last_date as YYYY-MM with the true last day zero-padded, first_day always equal to one, last_day equal to days_in_month, the month_name in English for display, is_leap_year for February-sensitive callers, and a short formula reminder. Rejected cases include a missing year or month, non-integer values, years outside one through 9999, and any month outside one through twelve such as zero, thirteen, or negative indices.

How year and month are read and how leap Februaries stay correct

Each value may arrive as a number, a pure integer string with an optional sign, or a bigint that fits the safe integer range; embedded decimals, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong last day. The preferred first field is year, but aliases y and Y are accepted so spreadsheets, query strings, and typed forms can reuse the same contract without renaming columns. The preferred second field is month, with aliases m, M, and mon for the same reason. After parsing, year must fall between 1 and 9999 inclusive, and month must fall between 1 and 12 inclusive — values such as 0, 13, or 2024 as a month fail with invalid_input naming the month field. Month lengths come from a fixed table of thirty-one, thirty, or twenty-eight days, except February which becomes twenty-nine when the year is a Gregorian leap year: divisible by four, except century years that are not divisible by four hundred. That rule correctly gives 2024-02-29 and 2000-02-29 while rejecting any attempt to invent thirty-two days in any month. First day is always one; last day equals days_in_month; both bounds are formatted with four-digit years and two-digit months and days. Pair the free browser run on this page with the priced API when you need the identical month bounds inside an automated pipeline.

Where month bounds fit in billing, reporting, and software

On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a one-off check to a pipeline without changing the mathematical result.