Date Calculator
Add or subtract days from a date.
Date Calculator: Add & Subtract Dates
Everything you need to know
About the Date Calculator
Managing dates and deadlines is a constant challenge in both personal and professional life. Our date calculator simplifies complex date arithmetic by handling varying month lengths, leap years, and calendar quirks automatically.
What you can do:
- Add time to a date: "What date is 90 days from today?"
- Subtract time from a date: "What was the date 6 months ago?"
- Find duration: "How many days between these two dates?"
- Count business days: Exclude weekends for project planning
- Count down to events: Days until a deadline, birthday, or holiday
Date Addition and Subtraction
Adding Days, Weeks, Months, and Years
Simply enter your starting date and the amount of time to add:
Example: Add 45 days to March 15, 2026
- March has 31 days
- March 15 + 16 days = March 31
- Remaining 29 days in April
- Result: April 29, 2026
Example: Add 3 months to January 31, 2026
- January 31 + 1 month = February 28, 2026 (2026 is not a leap year)
- 1 month = March 28, 2026
- 1 month = April 28, 2026
- Result: April 28, 2026
Note: When adding months, the calculator adjusts for month-end dates (e.g., Jan 31 + 1 month = Feb 28/29).
Subtracting Time from a Date
Example: Subtract 60 days from July 4, 2026
- July 4 - 4 days = June 30
- Remaining 56 days from June
- June has 30 days, so subtract 30 = May 31
- Remaining 26 days from May
- May has 31 days, so 31 - 26 = May 5
- Result: May 5, 2026
Duration Between Two Dates
Calculate the exact time span between any two dates:
Example: From January 1, 2024 to December 31, 2024
- Total days: 366 (2024 is a leap year)
- Total weeks: 52 weeks and 2 days
- Total months: 11 months and 30 days, or exactly 12 months
- Total years: 1 year
Example: From March 15, 2025 to August 22, 2025
- Full months: March 15 → April 15 → May 15 → June 15 → July 15 → August 15 (5 months)
- Remaining days: August 15 → August 22 (7 days)
- Duration: 5 months, 7 days
- Total days: 160
Business Days Calculation
For project management and legal deadlines, count only weekdays (Monday-Friday), excluding weekends and optionally holidays:
Example: Business days from Monday, June 1 to Friday, June 12
- Calendar days: 12
- Weekends: June 6-7 (Saturday, Sunday)
- Business days: 10
With holidays: If June 8 is a holiday
- Business days: 9
Real-World Date Calculations
| Scenario | Calculation | Example |
|---|---|---|
| Project deadline | Start date + duration | Contract signed March 1, 90-day project = May 30 |
| Return policy | Purchase date + window | Bought Jan 15, 30-day return = Feb 14 |
| Prenatal appointments | LMP + gestational age | 12-week scan = LMP + 84 days |
| Lease end | Start date + lease term | 12-month lease from July 1 = June 30 next year |
| Warranty expiration | Purchase + warranty period | 2-year warranty from Sept 1, 2024 = Aug 31, 2026 |
| Retirement date | Birth date + retirement age | Born May 10, 1960, retire at 67 = May 10, 2027 |
| Statute of limitations | Incident date + time limit | Accident Jan 1, 2-year limit = Jan 1, 2027 |
| Pregnancy due date | LMP + 280 days | LMP March 15 = Due Dec 22 |
Day of the Week
The calculator can also tell you what day of the week a date falls on:
Quick reference (2026):
- New Year's Day 2026: Thursday
- Valentine's Day 2026: Saturday
- Independence Day 2026: Saturday
- Christmas 2026: Friday
Julian Day Number
For astronomers and historians, the calculator provides the Julian Day Number (JDN)—a continuous count of days since January 1, 4713 BC:
- January 1, 2000 = JDN 2,451,545
- January 1, 2026 = JDN 2,460,478
Week Number
The ISO week date system is used in business and manufacturing:
- Week 1 is the week containing the first Thursday of the year
- Each week begins on Monday
- There are either 52 or 53 weeks in a year
Frequently Asked Questions
How do you calculate days between two dates?
Convert both dates to Julian Day Numbers and subtract: Days = JDN₂ - JDN₁
What happens if I add 1 month to January 31?
The calculator adjusts to the last day of the resulting month: January 31 + 1 month = February 28 (or 29 in leap years).
How many days are in a year?
365 in a common year, 366 in a leap year. On average, 365.2425 days accounting for leap year rules.
Can I calculate dates in the past?
Yes. Enter any past date and add or subtract time, or calculate the duration from that date to today.
What is the difference between calendar days and business days?
Calendar days include all days of the week. Business days exclude Saturdays, Sundays, and optionally holidays.