Date Calculator
Find the exact time between two dates — years, months, days, total days, weeks and business days — or add and subtract years, months, weeks and days from any date. Everything runs in your browser; nothing is uploaded.
Time between two dates
Add or subtract from a date
How the date calculator works
This page has two calculators. The first finds the duration between two dates: it works out the whole years, months and days between them, the total number of days and weeks, and how many of those days are weekdays. The second adds or subtracts a span — any combination of years, months, weeks and days — from a starting date and tells you the resulting date and the day of the week it falls on. Both use the calendar directly, so leap years, the different lengths of each month, and month-end clamping are all handled automatically.
Dates are read at local midnight, which means time zones and daylight-saving changes can never nudge a result by a day. The years/months/days breakdown always obeys one rule: start date + the result = end date exactly. That is why 31 December to 1 March is reported as "2 months and 1 day" rather than a raw subtraction that could produce a nonsensical negative day.
Business days, weekends and how they're counted
"Weekdays" counts only Monday through Friday inside the range; "Weekend days" counts the Saturdays and Sundays. Together they always add up to the day total for the same window. The count is exact for spans of any length because it is based on the day of the week of each endpoint and the number of whole weeks between them, not an approximation. One thing it deliberately does not do is remove public holidays: holidays differ by country, state and year, so to get true working days you would subtract the holidays that land on a weekday in your own region.
Inclusive vs. exclusive counting
The most common source of "off-by-one" confusion in date math is whether both endpoints are counted. Exclusive counting — the default here — gives the plain difference between the dates: 1 January to 2 January is 1 day. Inclusive counting treats both the first and last day as full days, so the same pair becomes 2 days. Rental days, hospital stays and some notice periods are counted inclusively; simple "how many days until" arithmetic is exclusive. The checkbox above switches between them.
Duration units reference
| Unit | Days | Notes |
|---|---|---|
| Day | 1 | The base unit — one calendar day. |
| Week | 7 | Seven days; fixed, never varies. |
| Fortnight | 14 | Two weeks — common for pay periods and notice. |
| Month | 28–31 | Varies by month (Feb 28/29, Apr/Jun/Sep/Nov 30, the rest 31). |
| Quarter | 90–92 | Three months (a business/fiscal quarter). |
| Common year | 365 | A non-leap year. |
| Leap year | 366 | Has 29 February; occurs roughly every 4 years. |
Leap years and month lengths
A year is a leap year (366 days, with 29 February) if it is divisible by 4 — except century years, which must also be divisible by 400. So 1700, 1800 and 1900 were common years while 1600 and 2000 were leap years; the next century year to skip a leap day is 2100. Because months are 28, 29, 30 or 31 days long, the calculator always uses the real length of the relevant month rather than a flat average — which is exactly why month-end dates clamp correctly.
| Month | Days |
|---|---|
| January | 31 |
| February | 28 (29 in a leap year) |
| March | 31 |
| April | 30 |
| May | 31 |
| June | 30 |
| July | 31 |
| August | 31 |
| September | 30 |
| October | 31 |
| November | 30 |
| December | 31 |
Common uses
- Counting down to a deadline, launch, exam, due date or event.
- Working out a notice period, probation end, or contract term ("90 days from signing").
- Finding how many working days are in a project window before adding your local holidays.
- Checking anniversaries, milestones and "how long ago was…" questions.
- Shifting a date by a fixed offset — a follow-up appointment, a warranty expiry, a renewal date.
Privacy
The calculator is entirely client-side. The dates you enter are computed in your own browser, are never sent to a server, and are not stored or logged — so it is safe for private planning, and it works offline once the page has loaded.
Frequently asked questions
- How do I calculate the number of days between two dates?
- Enter the earlier date in "Start date" and the later date in "End date" in the first calculator. It instantly shows the exact difference as years, months and days, plus the total number of days, total weeks, and how many of those days are weekdays (Monday–Friday). By default the count is exclusive — it is the plain difference between the two dates (the day after the start up to and including the end), which is what "days between" normally means. Tick "Include both the start and end day" to count them both, which adds one to the totals.
- What is the difference between "days between" and "days including both dates"?
- There are two common conventions. Exclusive counting gives the difference between the dates — the number of nights, or complete days, from one to the other; the gap between 1 January and 2 January is 1 day. Inclusive counting treats both endpoints as whole days, so 1 January to 2 January is 2 days. Neither is "more correct" — pick the one your context needs. Notice periods and rental days are often inclusive; simple date arithmetic is usually exclusive. This tool defaults to exclusive and lets you switch with a single checkbox.
- How are the years, months and days worked out?
- The calculator takes the most whole calendar months that fit between the two dates without overshooting the end date, then counts the remaining days. Adding a month lands on the same day number in the next month, or the last day of that month when it is shorter — so 31 January plus one month is 28 February (or 29 in a leap year). This "calendar" method is why 31 December to 1 March comes out as 2 months and 1 day rather than a raw column subtraction that could go negative, and it always satisfies the rule that start date + the result = end date exactly.
- How does it count business days (working days)?
- Business days are the days that fall on Monday through Friday within the range; Saturdays and Sundays are excluded. The count is exact — the tool uses the day of the week of each endpoint and the number of whole weeks in the span, so it never miscounts across long ranges. Note that it does not remove public holidays, which vary by country, state and year; if you need working days net of holidays you would subtract the holidays that fall on weekdays in your region.
- How do I add or subtract days, weeks, months or years from a date?
- Use the second calculator. Enter a start date, type how many years, months, weeks and/or days to shift by (you can combine them), choose Add or Subtract, and it shows the resulting date and its day of the week. Years and months are applied first using calendar arithmetic with end-of-month clamping (so 31 January minus one month is 28 February), and weeks and days are applied afterwards. This answers questions like "what date is 90 days from today?", "when is 6 weeks after my appointment?", or "what was the date 18 months ago?".
- Does it handle leap years correctly, and are my dates private?
- Yes to both. Every calculation uses the calendar directly, so it accounts for leap years (29 February), the different lengths of each month, and month-end clamping automatically. Dates are parsed at local midnight so time zones and daylight-saving changes never shift a result by a day. And the whole tool is plain JavaScript running in your browser — the dates you enter are never uploaded, stored or logged, and it keeps working offline once the page has loaded.