How do I add days to a date?

To add days to a date, count forward from the start date, accounting for the varying lengths of months and leap years. For simple additions (fewer than 30 days), count on a calendar. For larger additions, use the date calculator above — enter your start date and the number of days, weeks, months or years to add.

What date is 90 days from today?

To find the date 90 days from today, use the date calculator above and enter today's date with +90 days. The result changes daily. Common uses for 90-day calculations include contract periods, warranty deadlines, visa durations and return policy cut-offs.

How do I calculate an expiry or deadline date?

Enter the start date (e.g. contract signing date or purchase date) and add the relevant duration (e.g. 30 days, 6 months, 1 year) using the date calculator. The result is the exact expiry or deadline date. Useful for warranties, contracts, subscriptions and legal deadlines.

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Adding and Subtracting Dates: The Edge Cases That Bite

Built and verified by Andrius R. · Updated June 2026

Adding 30 days to a date is mechanical. Adding "one month" is a negotiation — because months aren't a fixed length, and the calendar is full of dates that don't exist. Here's how date arithmetic actually works, and where deadlines hide their traps.

Days are exact; months are a convention

The famous edge case — 31 January + 1 month

February has no 31st. Every system must pick a rule, and the near-universal convention (this calculator included) is clamp to the month's last day: 31 Jan + 1 month = 28 Feb (29 in a leap year). The asymmetry that follows surprises people: 31 Jan + 1 month + 1 month = 28 Feb + 1 month = 28 Mar — but 31 Jan + 2 months = 31 Mar. Adding months one at a time is not the same as adding them all at once. The same clamping handles 31 → 30-day months (31 May + 1 month = 30 Jun) and leap-day birthdays (29 Feb + 1 year = 28 Feb).

"Within 30 days" vs "within a month" — they differ, legally

A returns window of 30 days from 30 January ends 1 March (29 Feb in leap years); "one month" from the same date ends 28/29 February — the month version is shorter across February and longer across 31-day months. Contracts, visas and warranties pick one wording deliberately. Two more conventions worth knowing: whether the start day itself counts (most legal systems exclude the trigger day — "30 days from receipt" starts counting tomorrow), and whether a deadline landing on a weekend rolls forward (court rules usually yes, airline fare rules usually no). When real money or rights ride on a date, compute it exactly with the tool above and read which convention the document uses.

Business-day arithmetic

"10 business days from Thursday 11 June 2026" skips two weekends: land on Thursday 25 June — 14 calendar days for 10 working ones. The general shape: n business days ≈ n × 7/5 calendar days, plus holiday adjustments. Payroll, bank transfers ("funds available in 3–5 business days") and legal service periods all run on this clock, which is why a Friday-afternoon bank transfer feels slow: its "next business day" is Monday.

Why the calendar itself has scars

Date arithmetic inherits history's patches. The Gregorian reform (1582) fixed the Julian calendar's drift by deleting 10 days outright — adopted at different times by different countries, so Britain skipped 11 days in 1752 (Washington's birthday moved retroactively) and Russia not until 1918, which is why the "October Revolution" happened in November by our calendar. Software working with historical dates must choose a calendar; this calculator, like nearly all modern systems, uses the proleptic Gregorian calendar — today's rules extended backwards — which is consistent but means dates before a country's adoption won't match its contemporary documents.

Patterns for everyday use

Weekday of a future date: add the days, divide by 7, shift by the remainder — 100 days from a Thursday is 14 weeks (Thursday again) + 2 = a Saturday.
Recurring schedules: "every 4 weeks" and "monthly" drift apart by ~1–2 days per cycle and a full extra period per year (13 four-week cycles vs 12 months) — the root of many billing surprises.
Countdown planning: for the gap between two known dates rather than offsets from one, the date difference calculator is the right tool; for "how long until," the countdown calculator runs live.

// 30 Days vs 1 Month

Adding "1 month" moves to the same day next month. Adding "30 days" may land on a different date — months have 28–31 days.

// End of Month

Adding months to 31 January → 28/29 February, not 31 February. The result is clamped to the last valid day of the target month.

// Business Use

Common deadlines: 30 days notice, 90-day probation, 6-month review. Use this calculator to find the exact calendar date.

// Leap Year

Adding 1 year to 29 February → 28 February in non-leap years. Adding 4 years → 29 February again.