Date Duration Calculator
Calculate the number of days between two dates, add or subtract days from any date, or count business days. Handles leap years and variable month lengths accurately.
How to Use the Date Duration Calculator
This calculator provides three modes to handle every common date calculation:
- Days Between Dates — Select a start date and an end date to see the total calendar days, the equivalent in weeks, months, and a year/month/day breakdown. You also get the number of business days (weekdays only) and weekend days.
- Add/Subtract Days — Pick a starting date, enter a number of days, and choose whether to add or subtract. The resulting date is displayed in multiple formats: full date with month name, ISO 8601, US format (MM/DD/YYYY), and the day of the week.
- Business Days — Enter two dates to find exactly how many Monday-through-Friday business days fall between them, along with the count of weekend days, total calendar days, and full weeks.
Understanding Date Calculations
Counting days between dates requires careful handling of calendar irregularities. Here are the key factors this calculator accounts for:
Leap years add one extra day (February 29) every four years. However, century years (1900, 2100) are not leap years unless they are divisible by 400 (2000, 2400 are leap years). This means a span from January 1 to December 31 in a leap year covers 366 days instead of 365.
Business days vs. calendar days: A calendar day count includes every day regardless of weekday or weekend. Business days count only Monday through Friday. For a typical 30-day month, there are roughly 22 business days, but the exact number depends on which day of the week the month starts. Public holidays are not excluded because they vary by country and region.
Variable month lengths make "one month from today" ambiguous. January 31 plus one month could be February 28 or March 3 depending on interpretation. This calculator resolves this by computing months as full calendar months elapsed, with the remaining days counted separately, giving you an unambiguous result.
Common Date Calculations
This table shows frequently referenced date spans and their day counts for quick reference.
| Scenario | From | To | Days |
|---|---|---|---|
| US tax filing period | Jan 1 | Apr 15 | 105 |
| US school year (approx.) | Sep 1 | Jun 15 | 287 |
| Standard pregnancy | Conception | Due date | 280 |
| 90-day visa | Entry date | +90 days | 90 |
| 180-day Schengen limit | First entry | +180 days | 180 |
| US 30-day return policy | Purchase | +30 days | 30 |
| One calendar year | Jan 1 | Dec 31 | 365 |
| Leap year | Jan 1 | Dec 31 | 366 |
| Half year | Jan 1 | Jul 1 | 181 |
| Quarter (Q1) | Jan 1 | Mar 31 | 90 |
Quick Reference: Days in Each Month
The number of days in each month of the Gregorian calendar. February has 29 days in leap years (years divisible by 4, except century years not divisible by 400).
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 / 29 | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
Frequently Asked Questions
Enter a start date and an end date in the "Days Between Dates" tab and click Calculate. The calculator instantly shows the total calendar days, an equivalent in weeks and remaining days, months and remaining days, the number of business days (excluding weekends), and a full breakdown in years, months, and days.
Yes. All calculations use proper calendar arithmetic that handles leap years correctly. A year is a leap year if it is divisible by 4, except for century years (divisible by 100) which are only leap years if also divisible by 400. So 2024 and 2028 are leap years, 1900 was not, and 2000 was.
Business days (also called working days or weekdays) are Monday through Friday, excluding Saturday and Sunday. The calculator iterates through each day in the range and counts only those that fall on a weekday. Note that public holidays are not excluded because they vary by country, state, and employer.
Yes. Use the "Add/Subtract Days" tab. Enter a starting date and the number of days you want to add or subtract, then select the operation (add or subtract) and click Calculate. The result is shown in multiple formats including the full date with month name, ISO 8601 format (YYYY-MM-DD), US format (MM/DD/YYYY), and the day of the week.
Go to the "Add/Subtract Days" tab. The start date defaults to today. Enter 90 in the "Number of Days" field, make sure "Add days" is selected, and click Calculate. The resulting date is displayed immediately. This is useful for visa deadlines, return policies, warranty expiration, and similar time-sensitive calculations.
The Gregorian calendar inherited its irregular month lengths from the Roman calendar. The original Roman calendar had 10 months. January and February were added later by King Numa Pompilius. Julius Caesar reformed the calendar in 46 BC, and Augustus later adjusted it. The result is 7 months with 31 days, 4 months with 30 days, and February with 28 (or 29 in a leap year).
A standard 365-day year contains approximately 261 business days (weekdays). The exact number ranges from 260 to 262 depending on which day of the week the year starts and whether it is a leap year. This calculation does not account for public holidays, which further reduce the number of actual working days.