Date Calculator
Runs in your browserCalculate the difference between two dates or add/subtract days, weeks, months, and years from a date.
Enter Dates
Runs entirely in your browser — nothing is uploaded
Runs in your browser
Next steps
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Runs entirely in your browser. No uploads. Your files stay private.
Understanding Date Calculations
Date Calculator runs two operations against the browser's Date object: a difference operation that returns the gap between two calendar dates, and an addition operation that pushes a base date forward or backward by a chosen amount of days, weeks, months, or years. Both run synchronously in your tab — there's no network round trip.
Day differences are computed by subtracting epoch milliseconds and dividing by 86,400,000 (the number of milliseconds in a 24-hour day). This is exact when both dates fall in the same time zone offset. Across daylight-saving boundaries the gap can be off by 1 hour, which can flip a result from 7.0 to 6.96 days; the tool rounds to whole days to absorb this.
Months and years in the difference output are approximations: months use 30.44 days (the average month length across a 400-year Gregorian cycle) and years use 365.25 days (accounting for the leap-year rule). For legal or contractual periods that need calendar-accurate month counts — for example, a six-month notice period — use the addition mode with months=6, since that walks calendar months exactly using Date.setMonth().
When adding months or years with Date.setMonth() or Date.setFullYear(), the runtime handles end-of-month overflow by rolling forward. So January 31 + 1 month becomes March 3 (or March 2 in leap years), not February 28/29. This is the JavaScript spec behavior and matches most spreadsheet apps.
The calculator handles leap years using the standard Gregorian rule — divisible by 4, except century years not divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. February 29 inputs are accepted only in valid leap years; in other years the browser silently clamps the date to March 1.
Negative differences (end before start) are detected and shown with a minus sign rather than throwing an error. This makes the tool usable for both 'how long until' and 'how long since' queries with the same input fields.
All inputs are processed locally; nothing about your dates is logged, transmitted, or stored. Refresh the page and the form clears.
Common Use Cases
01
Project timeline planning
Find the day count between kickoff and ship dates to size sprints, set milestones, and forecast deliverables.
02
Deadline calculation
Add a 30-day, 60-day, or 90-day window to a contract date to determine response deadlines and renewal triggers.
03
Contract expiration
Add a 12-month or 24-month term to a signing date to find the exact expiration calendar date, accounting for leap years.
04
Appointment scheduling
Add 6 weeks to a base date for follow-up medical or dental appointments using exact calendar arithmetic.
Frequently Asked Questions
In Difference mode, months and years are approximations based on average lengths (30.44 days per month, 365.25 days per year). For an exact calendar-month count, use Addition mode with months as the unit, since it walks Date.setMonth() one month at a time and respects each month's real length.
Yes. Both Difference and Addition modes accept any past, present, or future calendar date. Negative differences are detected and shown with a minus sign instead of an error.
Yes. The browser's Date object uses the proleptic Gregorian calendar, which applies the divisible-by-4-except-century rule. So 2024 has 366 days, 2025 has 365, and 2100 will have 365 even though it's divisible by 4.
No. The form state lives only in React component state for the lifetime of the tab. Nothing is written to localStorage, cookies, or any server log.
JavaScript's Date.setMonth() rolls forward when the target month doesn't have the source day. January 31 plus 1 month becomes March 3 (or March 2 in leap years), not February 28. This matches Excel's default EDATE behavior in most cases.
Day counts are rounded to whole days, which absorbs the 1-hour error introduced by DST transitions twice a year. If you need DST-aware second-precision math, use the Timestamp Converter and work with Unix epoch values directly.
The current build returns a formatted date but not a weekday label. To find the weekday, copy the result date into the Age Calculator, which reports the day-of-week using Date.getDay().
JavaScript Date supports approximately ±100,000,000 days from the Unix epoch (Jan 1 1970), which is roughly the years -271,821 to +275,760. Practical use is anywhere within recorded human history.
Because leap years contain 366 days. Adding 1 year via Date.setFullYear() preserves the calendar month and day; adding 365 days via day arithmetic shifts the calendar by one day in any year that crosses February 29.
Inputs are parsed at local midnight, so the calculation reflects your machine's time zone. If you're computing across continents, treat the dates as wall-clock dates rather than UTC instants.
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