Date calculator
Calculation settings
Date calculator guide
The date calculator quickly counts days between two dates. You can also add or subtract days, weeks, months, or years from a date.
- Count days：Count days between two dates
- Add days：Add days to a date
- Subtract days：Subtract days from a date
How to use the date calculator
Count days between two dates
- Open the Count days tab.
- Choose the start date, or use Today.
- Choose the end date, or use Today.
- Select Calculate duration to see the day count.
You can include the end date in the result by adding one day.
Add days to a date
- Open the Add days tab.
- Choose a start date.
- Enter years, months, weeks, and days.
- Select Calculate new date.
Subtract days from a date
- Open the Subtract days tab.
- Choose a start date.
- Enter years, months, weeks, and days.
- Select Calculate new date.
Why use a day-count calculator?
Manual date math is easy to get wrong because leap years and month lengths vary. This calculator handles those details automatically.
How to use this tool
Calculate the day difference between two dates at UTC midnight, or add and subtract nonnegative whole years, months, weeks, and days from a date.
Calculate days between two calendar dates or find target dates in the future or past. Ideal for project deadlines, contract expirations, anniversary tracking, and countdowns.
💡 3 Quick Steps to Get Started
Choose between "Duration between dates" or "Add/Subtract days from date".
Pick start and end dates directly on the interactive date picker.
See the exact number of calendar days, weeks, and day-of-week breakdown.
When it helps
Duration mode can include the end date and exclude selected weekdays. Add and subtract modes apply years and months first, then weeks and days.
How it works
The day difference is the UTC-midnight millisecond difference divided by 86,400,000. Including the end date adds or subtracts one day by direction, and weekday exclusions apply only in duration mode. Weeks are days ÷ 7 and displayed months are days ÷ 30.4375. Date offsets use JavaScript UTC rollover, so a month-end date can roll into the following month.
Worked example
Example: count days from 2026-01-01 to 2026-01-31, or add one month to 2026-01-31 and review the JavaScript rollover date.
Limits and data handling
The displayed month difference is only days ÷ 30.4375, not a count of complete calendar months. The tool does not handle times, time zones, daylight-saving changes, or public holidays. Month-end offsets follow JavaScript date rollover and can land in the next month.
Dates, excluded weekdays, and results are processed only in the current browser. They are not sent to the GoGo Tools backend or stored by the site, and analytics events do not contain entered dates or results.
Date, Time & Calendar Mathematics Guide
1. Calendar Mathematics: The Gregorian Reform & Leap Year Logic
The modern Gregorian calendar, promulgated by Pope Gregory XIII in 1582, resolved the cumulative seasonal drift of the Julian calendar. Because Earth’s tropical year is approximately 365.24219 days, adding a simple leap day every 4 years overcompensated by roughly 3 days every 400 years.
The definitive Gregorian leap year rule specifies: A year is a leap year if divisible by 4, except for century years (divisible by 100), which are only leap years if also divisible by 400 (e.g., the year 2000 was a leap year, whereas 1900 and 2100 are common years).
2. Business Days, Project Milestones, and Legal Deadlines
Project planning and contract law strictly differentiate between calendar days and working/business days. Calculating lead times across international supply chains requires excluding weekends and jurisdiction-specific public holidays.
In contract law and regulatory filings, statutory deadline rules frequently stipulate that if the final day of a notice period falls on a weekend or public holiday, the cutoff automatically extends to the next business day.
3. Universal Time (UTC), IANA Time Zones, and Daylight Saving Time
Global timekeeping anchors to Coordinated Universal Time (UTC). Converting time between international locations involves both baseline geographic offsets and seasonal Daylight Saving Time (DST) adjustments. DST transitions vary by country and year, requiring automated lookup against the standardized IANA Time Zone Database (Olson database).
4. Unix Epoch Timestamps and High-Precision Time Math
Computer operating systems track time as the elapsed milliseconds since the Unix Epoch (January 1, 1970, 00:00:00 UTC). Our computation engine executes temporal arithmetic on atomic timestamps, guaranteeing zero-drift accuracy across leap years and month-boundary transitions.
Feedback
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Send a suggestion or bug report for this tool.
FAQ
Date calculator: Which dates and offsets can I enter?
Choose valid dates. Years, months, weeks, and days in add or subtract mode are parsed as nonnegative whole numbers. Duration mode also lets you include the end date and select weekdays to exclude.
Date calculator: How are the duration and new date calculated?
Duration uses UTC-midnight dates. Include end adjusts a forward or backward difference by one day, while weekday exclusions affect duration mode only. Offsets apply years and months first, followed by weeks × 7 and days.
Date calculator: What month, time-zone, and holiday limits apply?
The displayed month difference is only days ÷ 30.4375, not a count of complete calendar months. The tool does not handle times, time zones, daylight-saving changes, or public holidays. Month-end offsets follow JavaScript date rollover and can land in the next month.
Date calculator: Are my dates uploaded or stored?
Dates, excluded weekdays, and results are processed only in the current browser. They are not sent to the GoGo Tools backend or stored by the site, and analytics events do not contain entered dates or results.
