Understanding your date result
Each output expresses the same interval in a different working unit. Choose the one that matches your task: calendar planning, payroll weeks, delivery estimates or long-range comparisons.
| Result | Convention | Typical use |
|---|---|---|
| Total days | End date included, start date excluded | Countdowns, day-precise intervals |
| Weeks | Complete 7-day weeks plus remainder days | Schedules, pregnancy-style week counts |
| Business days | Mon-Fri only; public holidays NOT excluded | Shipping and turnaround estimates (upper bound) |
| Years | Days ÷ 365.2425 (mean Gregorian year) | Long intervals, decimal-year comparisons |
| Resulting date + weekday | Exact day shift in UTC | Deadlines, 'what date is 90 days from now' |
- Business days exclude only Saturdays and Sundays. National and regional public holidays are not subtracted, because holiday calendars vary by jurisdiction — check your local calendar for deadline-critical counts.
- The day count convention excludes the start date and includes the end date; some legal or financial contexts count inclusively on both ends, giving one day more.
- Leap years are handled automatically because the arithmetic runs on real calendar dates; 2024 and 2028 are leap years, 2026 is not.
- Dates are computed at midnight UTC, so no daylight-saving shift can make a 'day' longer or shorter in this calculator.
What does a date calculator do?
A date calculator performs calendar arithmetic. In difference mode it counts the days between a start date and an end date — the count excludes the start date and includes the end date, so Monday to the following Monday is 7 days. In add-days mode it moves forward or backward from a start date by an exact number of days (enter a negative number to subtract) and reports the resulting date and its weekday.
Business days are the subset of elapsed days that fall on Monday through Friday. This is the common commercial convention for shipping estimates and turnaround times. Public holidays are not excluded, because holiday calendars differ by country, region and even employer — treat the business-day figure as an upper bound and subtract any holidays that apply to your situation.
The years figure divides the day count by 365.2425 — the average length of a year in the Gregorian calendar, which adds a leap day every 4 years except in century years not divisible by 400. This gives a consistent decimal-year measure regardless of which specific years the interval spans.
How to use this date calculator
- Choose a mode: "Days between two dates" for an interval, or "Add or subtract days" to project a date.
- In difference mode, pick the start and end dates and read the total days, weeks, business days and decimal years.
- In add-days mode, pick the start date and enter the number of days — use a negative number to go backwards in time.
- Read the resulting date and its weekday; for deadlines, remember that public holidays are not excluded from business days.
The formula behind date arithmetic
The day difference is the number of calendar days between the two dates (end minus start, shown as an absolute value). Business days are counted by stepping through each elapsed day and keeping those that fall Monday through Friday. Adding days multiplies the day count by 86,400,000 milliseconds and shifts the start date by that amount, which is exact because dates are handled at midnight UTC.
Worked example (difference): 1 January 2026 to 15 March 2026 is 73 days, which is 10 weeks + 3 days, contains 51 business days (Mon-Fri, holidays not excluded), and equals 73 ÷ 365.2425 = 0.20 years. Worked example (add days): 1 January 2026 + 90 days = 1 April 2026, a Wednesday.
Common mistakes
- Treating the business-day count as holiday-adjusted — it only removes weekends, so subtract any public holidays in your jurisdiction yourself.
- Off-by-one errors from counting conventions — this calculator excludes the start date and includes the end date, while some contracts count both ends inclusively.
- Estimating months as 30 days each — calendar months vary from 28 to 31 days, so convert long intervals with real dates, not month approximations.
- Forgetting leap years in manual arithmetic — an interval spanning 29 February is one day longer than the same month-and-day span in a common year.
- Adding weeks when days were intended — 90 days is 12 weeks and 6 days, not 3 calendar months.
Häufig gestellte Fragen
How many days are between two dates?
Count the calendar days from the start date (excluded) to the end date (included). For example, 1 January 2026 to 15 March 2026 is 73 days: the remaining 30 days of January, 28 days of February (2026 is not a leap year) and 15 days of March. This calculator performs the count exactly, including leap days when the interval spans a 29 February.
What counts as a business day?
In this calculator, a business day is any elapsed day that falls on Monday through Friday. Public holidays are not excluded, because holiday calendars differ by country, region and employer. For deadline-critical calculations, subtract the public holidays that apply in your jurisdiction from the business-day figure shown.
What date is 90 days from today?
Add exactly 90 calendar days to today's date — the calculator shifts the date by 90 × 24 hours and reports the resulting date and weekday. For example, 90 days after 1 January 2026 is 1 April 2026, a Wednesday. Note that 90 days is 12 weeks and 6 days, which is close to but not the same as 3 calendar months.
How do I subtract days from a date?
Choose the add-or-subtract mode and enter a negative number of days. Entering −30 with a start date of 15 March 2026 returns 13 February 2026. The weekday of the resulting date is shown alongside, which is useful for checking whether a back-dated deadline lands on a weekend.
Why does the years figure use 365.2425 days?
365.2425 days is the average year length in the Gregorian calendar, which adds a leap day every 4 years except in century years not divisible by 400 (2000 was a leap year; 1900 was not). Dividing a day count by this mean value gives a consistent decimal-year figure regardless of which particular years the interval happens to span.
Does the calculator handle leap years?
Yes — all arithmetic runs on real calendar dates, so a 29 February inside the interval is counted as a real day automatically. An interval from 1 February to 1 March is 29 days in a leap year such as 2024 and 28 days in a common year such as 2026.
Quellenangaben
- ISO 8601 — Date and time — Representations for information interchange. International Organization for Standardization.
- United States Naval Observatory / Astronomical Applications Department — Gregorian calendar leap-year rules.