Date Calculator — Add or Subtract Days, Months, Years from a Date

Both, depending on which arithmetic you ask for. The naive answer is Mar 2 (Jan 31 + 31 days = Mar 2 in a non-leap year, Mar 3 in a leap year), and that's what JavaScript's native `Date.prototype.setMonth(1)` produces — ECMA-262 §21.4.4.25 specifies that when the previously-set day exceeds the new month's days, the date overflows to the following month. The calendar-aware answer is Feb 28 (Feb 29 in a leap year): clamping the day to the last day of the target month. Most legal/financial contexts assume the calendar-aware clamping convention (a "monthly billing cycle" starting Jan 31 bills again Feb 28, Mar 31, Apr 30, May 31, Jun 30 — never spilling to "Feb 31" or rolling forward). This tool applies explicit clamping on top of `setMonth`. The TC39 Temporal API (Stage 3 as of 2026) provides `Temporal.PlainDate.with({day: 'last'})` for the same clamping idiom.

The Gregorian leap year rule, codified by Pope Gregory XIII's papal bull Inter gravissimas (24 February 1582), is: a year is a leap year if it is divisible by 4, EXCEPT if it is divisible by 100, EXCEPT if it is divisible by 400. Worked examples: 2024 ÷ 4 = leap. 1900 ÷ 4 leap, but ÷ 100 not leap, and not ÷ 400 → NOT a leap year. 2000 ÷ 4 leap, ÷ 100 not leap, but ÷ 400 leap → IS a leap year. 2100 ÷ 4 leap, ÷ 100 not leap, not ÷ 400 → NOT a leap year. The rule keeps the calendar's mean year at 365.2425 days, matching the tropical year to within ~27 seconds (drifts 1 day every ~3,225 years). Feb 29 birthdays observe Feb 28 on non-leap years for civil purposes (some jurisdictions use Mar 1). The 1582 reform also skipped 4–15 October 1582 to realign the vernal equinox; Britain followed in 1752 (skipping 11 days), Russia in 1918, Greece in 1923.

Naegele's rule, formulated by Franz Karl Naegele (Heidelberg professor of obstetrics, 1812), estimates a pregnancy's due date as LMP (last menstrual period) + 280 days = LMP + 40 weeks = LMP + 9 calendar months + 7 days. It assumes a 28-day menstrual cycle with ovulation on day 14, so it's a population-average heuristic. Modern obstetric practice (ACOG Committee Opinion 700, May 2017) confirms Naegele's rule as the starting point but recommends first-trimester ultrasound crown-rump length (CRL) measurement for redating if the ultrasound estimate differs from LMP-based dating by more than 5 days in trimester 1 or 7 days in trimester 2. Beyond 22 weeks, ultrasound dating becomes too imprecise to override LMP-based EDD. Practical use: enter your LMP in the date calculator, add 280 days, get the EDD; subtract 280 to back-calculate LMP from a known ultrasound EDD.

ISO 8601:2019 (YYYY-MM-DD) is the only widely-supported date format that meets two critical properties: it's locale-unambiguous (the US "02/03" reads as Feb 3, the EU "02/03" as March 2 — never the same conversation in ISO format) and it's lex-sortable as a plain string (so "2024-12-31" < "2025-01-01" sorts correctly without parsing). RFC 3339 (the IETF profile of ISO 8601, Klyne & Newman 2002) is mandated for JSON Schema, OpenAPI specifications, JWT iat/exp claims, HTTP Date headers, structured logs, and ISO TIMESTAMP database columns. The day-of-week shown alongside is computed by JavaScript's `Date.prototype.getDay()` which returns 0–6 (Sunday–Saturday) per ECMA-262. If you need a localised display format ("31 December 2024" or "31.12.2024"), do that conversion in user-facing UI just before render — never in storage, transmission, or interchange.

Common contract date conventions: "Net 30" = invoice issue date + 30 calendar days (not business days unless specified); some industries use Net 60, Net 90, or 2/10 Net 30 (2% discount if paid within 10 days, full balance due in 30). Lease "12-month term" = lease start date + 12 calendar months (same calendar day next year, clamped if the start day doesn't exist in the end month — a Feb 29 lease ends Feb 28). Insurance "1-year policy" = effective date + 1 calendar year, day-clamped. Mortgage payment due dates roll on the calendar day each month (a 31st-of-month mortgage clamps to month-end on shorter months — typically Feb 28/29 + Apr/Jun/Sep/Nov 30). Court filing deadlines typically count business days exclusive of weekends and public holidays, plus a "next business day" rule if the deadline falls on a weekend. Always check the contract's defined-terms section for exact "day" and "month" definitions — most jurisdictions default to calendar days but explicit specification overrides default.