📅 Date Calculator
Calculate Days Between Dates & Add/Subtract Days
Accurate • Leap Year Support
📊 Days Between Two Dates
Calculate the number of days, months, and years between any two dates.
➕ Add Days to Date
Calculate what date it will be after adding days, weeks, months, or years.
➖ Subtract Days from Date
Calculate what date it was before subtracting days, weeks, months, or years.
How Date Calculations Work
Date calculations are essential for many purposes including project planning, contract management, age calculation, and event scheduling. Accurate date calculations must account for varying month lengths, leap years, and the specific calendar system in use.
⭐ Date Calculation Method: Modern date calculations use the Gregorian calendar system, which accounts for leap years and varying month lengths. JavaScript Date objects handle all these complexities automatically.
Days Between Two Dates Formula
Days Between = (End Date - Start Date) ÷ (24 × 60 × 60 × 1000)
When calculating days between dates in JavaScript:
- Convert both dates to milliseconds since epoch (January 1, 1970)
- Subtract the start date from the end date
- Divide by the number of milliseconds in a day (86,400,000)
- Round to get the number of complete days
Adding/Subtracting Dates Formula
New Date = Original Date + (Years + Months + Days)
When adding or subtracting time periods, the calendar automatically handles:
- Month lengths (28-31 days)
- Leap years (February has 29 days)
- Year transitions
- Month/year rollovers
Understanding Leap Years
A leap year occurs every 4 years, except for years divisible by 100 (unless also divisible by 400). For example:
- 2024 is a leap year: Divisible by 4 (not a century year)
- 2000 was a leap year: Divisible by 400
- 1900 was NOT a leap year: Divisible by 100 but not 400
- 2100 will NOT be a leap year: Divisible by 100 but not 400
| Years | Leap Year Rule | Days in February | Examples |
|---|---|---|---|
| Divisible by 400 | ✓ Leap Year | 29 days | 1600, 2000, 2400 |
| Divisible by 100 (not 400) | ✗ Common Year | 28 days | 1700, 1800, 1900, 2100 |
| Divisible by 4 (not 100) | ✓ Leap Year | 29 days | 2004, 2008, 2012, 2016 |
| Not divisible by 4 | ✗ Common Year | 28 days | 2001, 2002, 2003, 2005 |
Practical Applications of Date Calculations
- Project Management: Calculating project duration and deadlines
- Contract Management: Determining contract end dates and milestones
- Financial Planning: Interest calculations and maturity dates
- Event Planning: Countdown to events and scheduling
- Appointment Scheduling: Follow-up dates and recall periods
- HR Management: Service anniversaries and benefits eligibility
- Age Verification: Determining eligibility based on date of birth
- Insurance: Premium periods and policy maturity
Month Lengths Reference
- 31 days: January, March, May, July, August, October, December
- 30 days: April, June, September, November
- 28 or 29 days: February (29 in leap years)
Days of the Week Calculation
When adding dates, it's also useful to know what day of the week the result falls on. The calculator automatically shows this using the Day Names (Monday, Tuesday, etc.).
Frequently Asked Questions
How accurate is this date calculator?
This calculator is very accurate. It uses JavaScript Date objects which properly handle leap years, varying month lengths, and all Gregorian calendar complexities. The accuracy depends on the date input being correct.
Does this calculator account for leap years?
Yes, absolutely. The calculator automatically accounts for leap years and correctly handles February 29 dates. It follows the Gregorian calendar leap year rules (divisible by 4, except century years unless divisible by 400).
What's the difference between counting days with or without the end date?
If you include the end date, it counts both the start and end dates in the total. For example, from Jan 1 to Jan 3: excluding end date = 2 days, including end date = 3 days (Jan 1, 2, and 3).
Can I calculate dates in the past or future?
Yes. You can enter any date in the past or future. The calculator works with dates from the year 1900 onwards (some browsers may support earlier dates). The calculations remain accurate regardless of the time period.
How does the calculator handle month boundaries?
The calculator properly handles month boundaries. When adding months to dates, it accounts for varying month lengths. For example, adding one month to January 31 correctly results in February 28/29.
What happens when I add days that cross month/year boundaries?
The calculator automatically handles these transitions. For example, adding 5 days to December 28 results in January 2 of the next year. All calculations handle these rollovers correctly.
Can I use this for business day calculations?
This calculator counts all days including weekends. For business days only (excluding weekends and holidays), you would need a specialized business day calculator that you can configure with your holidays.
Is there a limit to how far back or forward I can calculate?
JavaScript Date objects can handle dates from approximately year 1900 to year 9999. Most practical applications fall well within this range. The calculator will alert you if you try to use unsupported dates.
How is the day of the week determined?
The calculator uses the Gregorian calendar system to determine the day of the week. It accounts for the calendar cycle and any date shifts due to leap years and month lengths.
What year was the Gregorian calendar adopted?
The Gregorian calendar was introduced by Pope Gregory XIII in 1582. However, different countries adopted it at different times. This calculator uses the proleptic Gregorian calendar (applying current rules to all dates).
Official Date Calculation & Reference Sources
All date calculation information sourced from official standards and technical references:
📊 OmniCalculator - Days Between Dates 🕐 TimeAndDate - Date Duration Calculator 📅 Calendar Labs - Date Calculator 🧮 GIGACalculator - Add Days Calculator 📋 Microsoft - Date Calculation Methods 📚 Wikipedia - Leap Year Information