Date Add & Subtract Calculator
Add or subtract days, weeks, months or years from any date and get the resulting calendar date, weekday and a plain-language summary of the shift.
Leave a box empty to skip it. Years and months are applied first as calendar steps, then the weeks and days are counted on top.
Set a start date and an amount, then press Calculate Date.
Resulting date
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Where that lands
How the shift was applied
Quick navigation
Adding a month to a date sounds like it should be trivial. It is the single most error-prone operation in date arithmetic, and the reason is one sentence long: not every month has a 31st.
Take 31 January and add one month. There is no 31 February, so something has to give. There are three plausible answers and only one of them is actually used:
| Approach | 31 Jan + 1 month | Who does this |
|---|---|---|
| Clamp to last valid day | 28 or 29 February | Standard behaviour, SQL databases, spreadsheets, most date libraries, and this calculator |
| Overflow into the next month | 3 March (or 2 March in a leap year) | Naive implementations that increment the month number and leave the day alone |
| Refuse to answer | Error | A few strict libraries, on the grounds that the question is ill-posed |
The clamped answer is what people mean, which is why it won. This calculator flags every result where clamping occurred so you know it happened.
This is also why "the same date next month" is a phrase worth avoiding in contracts. A tenancy that begins on 31 August and runs "to the same date each month" has an ambiguous rent day in February, and the ambiguity is resolved differently by different letting agents. Specifying "the last day of each month" or "the 28th of each month" removes the problem entirely.
If you add both months and days, the order you apply them in changes the answer. This is not a bug in anyone's code, it is a genuine property of calendar arithmetic, which is not commutative.
Starting from 31 January 2026:
- Months first: 31 Jan + 1 month = 28 Feb (clamped), then + 30 days = 30 March.
- Days first: 31 Jan + 30 days = 2 March, then + 1 month = 2 April.
Three days apart, from the same inputs. The calculator applies years, then months, then weeks, then days, the order used by essentially every mainstream date library, and the order that matches how people describe spans out loud ("a year and two months and ten days"). If you need the other order, run the tool twice: shift by months first, take the result, then shift that by days.
Where it works well
- Years, months, weeks and days in one operation, with a stated order of application
- Month-end clamping done correctly, and flagged when it happens
- Works backwards as well as forwards with identical clamping rules
- Immune to daylight-saving shifts because it advances calendar dates, not millisecond offsets
- Shows the resulting weekday, which is usually the thing you actually needed
Where it falls short
- Calendar days only, weekends and holidays are not skipped
- Non-commutative by nature, so mixed month-and-day shifts depend on the documented order
- Clamped results cannot be reversed to recover the original day-of-month
- No support for recurring schedules; it computes one shifted date at a time
Most real uses of this tool fall into a handful of patterns. The trick in each case is knowing whether the period is defined in days or in months, because the two give different dates.
| Situation | Usually defined as | What to watch |
|---|---|---|
| Payment terms | 30, 60 or 90 days | Days, not months, "net 30" from 1 January is 31 January, not 1 February |
| Notice period | Weeks or months | Whether the clock starts on the day notice is given or the day after |
| Warranty or guarantee | Months or years | A 24-month warranty from 29 February expires 28 February in a non-leap year |
| Passport validity rule | 3 or 6 months beyond travel | Some countries measure from entry, some from exit, check before booking |
| Prescription or repeat | Days | Count from the dispensing date, and remember a 28-day supply is not a month |
| Trial or probation | Months | Clamping at month-end can shorten the final period by up to three days |
| Statutory limitation period | Years | Jurisdiction-specific and frequently subject to exceptions, confirm with the relevant rules |
Where a document says "one month" without further definition, the safer reading is usually the earlier of the two candidate dates.
Four weeks is not a month. It is a sentence worth repeating because the assumption is embedded in a surprising number of schedules.
Four weeks is 28 days. The average calendar month is about 30.44 days. The gap is roughly two and a half days per month, which compounds to thirteen four-week periods per year rather than twelve. Payroll systems that pay every four weeks therefore run thirteen cycles annually, which is why four-weekly-paid staff receive two payments in one calendar month once a year, and why a budget built on "four weekly payments equals monthly rent" fails.
| Period | Days | Cycles per year |
|---|---|---|
| One week | 7 | 52, plus one or two days |
| Two weeks (fortnight) | 14 | 26, plus one or two days |
| Four weeks | 28 | 13, plus one or two days |
| One calendar month | 28 to 31 | Exactly 12 |
| One quarter | 90 to 92 | Exactly 4 |
Only calendar months and quarters divide the year evenly. Everything measured in weeks leaves a remainder, which is why week-based and month-based schedules drift apart.
The calculator keeps weeks and days as separate fields precisely so you can express the period the way your document does. If a contract says "six weeks", put 6 in the weeks field rather than 42 in the days field, the result is identical, but the intent is preserved if you come back to it.
Three quick checks catch nearly every error in a shifted date, and all three take a few seconds.
Check the weekday. Adding a multiple of seven days must land on the same weekday you started from. If you added 28 days and the weekday changed, something is wrong. This is why the calculator always shows the resulting weekday.
Check the direction. Adding should move forward, subtracting back. It sounds obvious, but the most common real-world error with these tools is leaving the direction on the wrong setting and getting an answer that is plausible, wrong, and roughly twice the intended distance away.
Check whether clamping occurred. If the calculator flags a clamped result and your period is legally or financially significant, that is the moment to go and read the exact wording of the document rather than trusting either interpretation.
To verify a shifted date independently, put the original and the result into the Date Difference Calculator, the span it reports should match what you asked for. If you are working towards a business deadline rather than a calendar one, the Working Days Calculator is the tool that skips weekends and holidays for you.