First and Last Day of Month Calculator – Instant Billing and Payroll Boundaries
Knowing exactly which dates bound a "month" sounds trivial until you are closing monthly books, setting a subscription renewal date, or defining an SLA window that must end on the correct calendar day. The First and Last Day of Month Calculator removes the guesswork by instantly returning the first and last calendar date of any month, for any year, along with the weekday each one falls on — handling leap-year February and every other month length automatically.
How the Calculation Works
The first day of a month is always day 1. The last day depends on the month: most months run to 30 or 31, while February ends on 28 in a common year and 29 in a leap year. A year is a leap year under the modern Gregorian calendar if it is divisible by 4, except century years, which must also be divisible by 400 — so 2028 and 2032 end February on the 29th, while 2026 and 2100 end on the 28th. The tool applies this rule automatically so results are exact for any year from 1 to 9999.
Three Ways to Pick a Month
You can choose the month directly from dropdowns, enter a relative offset such as -1 for last month or +3 for three months from now, or simply type any date that falls inside the month you care about and let the tool derive the containing month's boundaries automatically. Only one mode is active at a time, so the input stays unambiguous no matter which approach you prefer.
Business Day Mode
Many billing cycles and payroll cutoffs don't actually land on a calendar boundary — they need the nearest working day. Toggling Business Day Mode calculates the first and last working day of the month by skipping Saturdays and Sundays, and optionally any dates you list in the holiday exclusion field, so a due date that would otherwise fall on a weekend automatically shifts to the nearest valid business day within the month.
ISO Week Numbers and Output Formats
Each result also includes the ISO 8601 week number containing the first and last day, which is useful for teams that plan work in week-based sprints rather than pure calendar months. Results can be displayed in ISO 8601 (YYYY-MM-DD), US (MM/DD/YYYY), EU (DD/MM/YYYY), or long-form (e.g. "Wednesday, July 1, 2026") formats, and copied to your clipboard with a single click.
Generating a Full Year of Billing Periods
Switch to the Batch tab to generate first/last day pairs for a whole run of consecutive months in one step — enter a starting month and year plus how many months you need (up to 60), and the tool builds a table showing every month's boundaries, weekdays, and total day count. The table can be exported as a CSV file, which is a fast way to seed a spreadsheet with a year's worth of billing periods, pay cycles, or reporting windows without typing each date by hand.
Who Uses This Tool
Finance and accounting teams use it to close monthly books on the correct date. Payroll and HR teams use it to set pay period boundaries. Project managers use it to define monthly milestones, and developers and QA engineers use it as a quick reference when testing date-range logic. Because the calendar date bounding a month is the same everywhere in the world, no time zone conversion is needed — the tool works purely on the calendar date.