Date Calculator - Add, Subtract, and Count Days
Need to know what date it will be 90 days from now? Or how many days between two dates? This calculator handles everything - date math, business days, week numbers, and leap year adjustments.
What This Calculator Does
- Add or subtract days, weeks, months, or years from any date
- Count days between two dates (with or without weekends)
- Calculate business days (excluding weekends and holidays)
- Find the day of the week for any date
- Convert between date formats
Common Date Calculations
| Duration | Example |
|---|---|
| 30 days from today | One month ahead |
| 90 days from today | Quarter ahead |
| 180 days from today | Half year ahead |
| 365 days from today | One year ahead |
| 10 business days | Two work weeks |
| Business days Q1 | ~65 days (Jan-Mar) |
Worked Examples
Example 1: 90 days from January 15, 2026 = April 15, 2026
Example 2: Days between March 1 and December 31, 2026 = 305 days
Example 3: 20 business days from Monday, October 5, 2026 = Monday, November 2, 2026 (skipping 4 weekends)
Calendar Rules to Know
- Leap years: Every 4 years, except centuries not divisible by 400
- Month lengths: 28-31 days (February is 28 or 29)
- DST changes: US observes daylight saving time (2nd Sunday March to 1st Sunday November)
- Week numbers: ISO 8601 starts weeks on Monday
- Business days: Monday-Friday, excluding federal holidays
US Federal Holidays (2026)
- New Year's Day: January 1
- MLK Day: January 19
- Presidents Day: February 16
- Memorial Day: May 25
- Juneteenth: June 19
- Independence Day: July 4
- Labor Day: September 7
- Columbus Day: October 12
- Veterans Day: November 11
- Thanksgiving: November 26
- Christmas: December 25
Frequently Asked Questions
How do I add days to a date?
Use the calculator's "Add/Subtract" mode. Enter the start date and number of days.
How many days are in each month?
January 31, February 28/29, March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30, December 31.
What is a business day?
Typically Monday-Friday excluding federal holidays. Some businesses use different calendars.
How do I count days excluding weekends?
Use the "business days" mode in the calculator. It skips Saturday and Sunday automatically.
What is the ISO week number?
A standard week numbering system where weeks start on Monday. Week 1 is the week containing the first Thursday of the year.
What is the difference between a calendar day and a business day?
Calendar day includes weekends and holidays. Business day excludes Saturdays, Sundays, and typically holidays.