Date Duration Calculator
Calculate the exact duration between two dates — or add and subtract time from any date. Results shown in years, months, weeks, days, hours, minutes, and seconds. Free, instant, works with any date range including historical dates.
Result date
How to Use the Date Duration Calculator
- Choose mode: Between Dates to find the duration, or Add / Subtract to find a resulting date.
- Enter your dates (and optional times) in the date fields.
- Results update instantly — years, months, weeks, days, hours, minutes, and seconds.
Why Date Duration Calculations Are Tricky
Date arithmetic looks simple — subtract one date from another — but it hides layers of complexity. Months have different lengths: February has 28 or 29 days; others have 30 or 31. "One month from January 31" is genuinely ambiguous since February 31 does not exist. Leap years add a day to February every four years (with century-year exceptions). Daylight Saving Time shifts clocks forward or back, creating days with 23 or 25 hours.
This calculator uses calendar-correct month counting: it advances month by month rather than multiplying by an average. Remaining days are counted exactly. This gives the most intuitive result for questions like "how old am I?" or "how long until my contract ends?"
The Add / Subtract mode solves the inverse problem: given a start date and a duration, what is the end date? This is useful for deadline planning ("what date is 90 days from today?"), contract management ("when does a 6-month term end?"), and event scheduling.
Frequently Asked Questions
How do I calculate the number of days between two dates? â–¼
Enter a start date and an end date. The calculator subtracts the earlier date from the later one to give you the total days, then breaks the result down into years, months, weeks, and days. Time components (hours, minutes, seconds) are also shown if you enable time inputs.
Does the calculator include both start and end dates? â–¼
The default mode counts the number of elapsed days — it does not include the start date itself. For example, from Jan 1 to Jan 3 is 2 days. If you want to include both endpoints (e.g., for event durations like "Jan 1 through Jan 3 = 3 days"), add 1 to the result.
How do I add days to a date? â–¼
Switch to the "Add / Subtract" tab, enter your start date and the number of days (or weeks, months, or years) you want to add. The result shows the target date. Use a negative number to subtract.
How are months calculated between two dates? â–¼
Month differences are calculated by counting full calendar months, then adjusting for remaining days. For example, from Jan 15 to Mar 20 is 2 months and 5 days — not exactly 2.17 months. This matches how most people intuitively count "how many months ago was it."
Why do different calculators give different results for months? â–¼
Months have different lengths (28–31 days), so "one month" is ambiguous as a fixed number of days. Some calculators use 30.44 days as an average month; others count calendar months. This calculator counts full calendar months with exact remaining days — the most readable approach.
Can I calculate time in hours or minutes? â–¼
Yes. Enable the time inputs to include hours, minutes, and seconds in your start and end dates. The calculator will show the total duration to the second.
What is a date duration used for? â–¼
Common uses: counting days until a deadline or event, calculating your exact age in days, finding project duration, computing contract lengths, calculating how long ago a date was, and planning countdowns.
Does this work with dates before 1970? â–¼
Yes. The calculator handles dates across all years. Internally it uses JavaScript Date objects which support dates from year 100 to 9999, so historical date ranges (e.g., "how many days between July 4, 1776 and today") work correctly.