How to Use the Date Calculator
- Date difference: Select two dates and the calculator shows the exact difference in years, months, weeks, and days.
- Add/subtract days: Enter a starting date and the number of days to add or subtract to find a future or past date.
Common Uses for a Date Calculator
- Find how many days remain until a deadline, event, or holiday.
- Calculate how long ago something happened (project start, anniversary, birth).
- Count the exact days between two contract dates or billing periods.
- Add 30, 60, or 90-day periods to a purchase or contract start date.
- Calculate project duration or time elapsed since a milestone.
- Determine if a given period spans a specific number of calendar months.
How Date Differences Are Calculated
Date arithmetic sounds simple but has subtle complexities:
- Leap years: February has 28 or 29 days depending on the year. The calculator handles this automatically using JavaScript's native Date object.
- Month lengths vary: Months have 28, 29, 30, or 31 days. "One month" is not always 30 days — it depends on which month you're counting.
- Day counting convention: The difference calculator counts the total calendar days between the two dates. Start date is typically not counted; end date is included (exclusive start, inclusive end).
Business Days vs. Calendar Days
This calculator counts all calendar days including weekends and holidays. If you need to count business days (Monday–Friday only, excluding holidays), note that the result here will be higher than the actual number of working days. For example, 10 calendar days spans approximately 7 business days if no holidays fall in that period.
Frequently Asked Questions
Does it count the start and end date?
The difference calculator counts calendar days from start to end. The start date is not counted as a day elapsed; the end date is. This matches how most deadline and duration calculations work — if something starts on the 1st and ends on the 10th, that's a 9-day duration.
Are leap years accounted for?
Yes. The calculator uses JavaScript's native Date object, which correctly handles leap years, varying month lengths, and daylight saving time transitions.
Can I calculate a date far in the past or future?
Yes. JavaScript's Date object supports dates from roughly 271,821 BCE to 275,760 CE. Practically any historical or future date you'd need is supported.
How do I find a date 90 days from today?
Use the "Add days" mode. Set the starting date to today and enter 90 in the days field. The result shows the exact date 90 calendar days from now.