Date Calculator
Calculate the days between two dates, or add/subtract days from any date. Results include weeks, months, and years.
How the Date Calculator Works
This calculator handles two common date tasks:
- Days Between: Enter a start date and end date. The calculator returns the total days, plus a breakdown in weeks, months, and years.
- Add / Subtract Days: Enter a start date and a number of days to add (or subtract, using a negative number). The calculator returns the resulting date and day of the week.
The math behind it
Every date can be converted to a Julian Day Number — a continuous count of days since a fixed reference point. Once both dates are expressed as day numbers, the difference is simple subtraction:
- Days between: |DayNumber(end) − DayNumber(start)|
- Add days: DayNumber(start) + N → convert back to calendar date
The calculator automatically accounts for:
- Months with different lengths (28, 29, 30, or 31 days)
- Leap years (February 29 in years divisible by 4, except centuries not divisible by 400)
- Day-of-week calculation (Monday through Sunday)
Quick reference: common intervals
| Interval | Days | Weeks | Common use |
|---|---|---|---|
| 1 week | 7 | 1 | Weekly recurring events |
| 2 weeks (biweekly) | 14 | 2 | Pay periods |
| 1 month (approx) | 30–31 | ~4.3 | Monthly billing |
| 90 days | 90 | ~12.9 | Quarterly deadlines |
| 180 days | 180 | ~25.7 | Half-year planning |
| 1 year | 365 (366 leap) | 52.1 | Annual events |
| 100 days | 100 | ~14.3 | Project milestones |
| 1,000 days | 1,000 | ~142.9 | Long-term goals |
Date Calculation Tips
Mental math shortcuts
- Same month: Just subtract the day numbers. March 20 − March 5 = 15 days.
- Adjacent months: Count remaining days in the first month, then add days in the second. Jan 25 to Feb 10 = 6 (rest of Jan) + 10 (Feb) = 16 days.
- Quick week estimate: Divide total days by 7. 90 days 7 ≈ 12 weeks and 6 days.
- Day-of-week trick: Each non-leap year advances the day of week by 1 (365 = 52×7 + 1). A leap year advances it by 2. So if January 1, 2025 is a Wednesday, January 1, 2026 is a Thursday.
Leap year rules
A year is a leap year if:
- It is divisible by 4 — but
- If it is divisible by 100, it is not a leap year — unless
- It is also divisible by 400, then it is a leap year
Examples: 2024 is a leap year (÷4). 1900 is not (÷100 but not ÷400). 2000 is a leap year (÷400).
Business days vs calendar days
Business days exclude weekends (and sometimes holidays). A quick estimate:
- 5 business days per 7 calendar days ≈ 71.4%
- For a 30-day period: roughly 21–22 business days
- For a 90-day period: roughly 64–65 business days
For exact business day counts, you need to know which days of the week your range starts and ends on.
Worked Examples
Example 1: Days between two dates
How many days are there from January 1, 2025 to December 31, 2025?
- 2025 is not a leap year, so it has 365 days
- January 1 to December 31 = 364 days (difference, not counting both endpoints)
- In weeks: 364 ÷ 7 = 52 weeks exactly
This is why New Year's Day advances by one day of the week each non-leap year.
Example 2: What date is 100 days from today?
If today is March 1, 2026, what date is 100 days later?
- March has 31 days → 30 remaining days (March 2–31)
- April has 30 days → 30 + 30 = 60 days (end of April)
- May has 31 days → 60 + 31 = 91 days (end of May)
- Need 9 more days → June 9, 2026
100 days from March 1, 2026 = June 9, 2026 (a Tuesday).
Example 3: How old am I in days?
A person born on July 4, 1990 — how many days old are they on January 1, 2026?
- From July 4, 1990 to July 4, 2025 = 35 years
- 35 years × 365 = 12,775 days + 9 leap days (1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024) = 12,784 days
- July 4, 2025 to January 1, 2026 = 27 (rest of Jul) + 31 (Aug) + 30 (Sep) + 31 (Oct) + 30 (Nov) + 31 (Dec) + 1 (Jan 1) = 181 days
- Total: 12,784 + 181 = 12,965 days
Real-World Date Applications
Date calculations appear in many everyday situations. Here's how to handle common scenarios:
Project deadlines and milestones
Your project starts on March 15 and has a 90-day deadline. When is it due?
- March 15 + 90 days
- March has 31 days → 16 remaining days (March 16–31)
- April has 30 days → 16 + 30 = 46 days
- May has 31 days → 46 + 31 = 77 days
- Need 13 more days → June 13
90 days from March 15 = June 13 (a Friday).
Pregnancy due date calculation
Pregnancy typically lasts 280 days (40 weeks) from the first day of the last menstrual period. If the LMP was January 10:
- January 10 + 280 days
- Using Naegele's rule: add 7 days, subtract 3 months, add 1 year
- January 10 + 7 days = January 17
- January 17 − 3 months = October 17
- October 17 + 1 year = October 17 of the same year
Due date: October 17 (a Tuesday, assuming a non-leap year).
Warranty and subscription expiration
You bought a laptop on November 22, 2025 with a 1-year warranty. When does it expire?
- November 22, 2025 + 1 year = November 22, 2026
- 2026 is not a leap year, so the warranty covers exactly 365 days
- In weeks: 365 ÷ 7 = 52 weeks and 1 day
Warranty expires: November 22, 2026 (a Sunday).
Event countdown planning
You're planning a wedding for September 20, 2026. Today is February 1, 2026. How many days do you have?
- February 1 to September 20, 2026
- February: 27 remaining days (2026 is not a leap year)
- March: 31 days → 27 + 31 = 58
- April: 30 days → 58 + 30 = 88
- May: 31 days → 88 + 31 = 119
- June: 30 days → 119 + 30 = 149
- July: 31 days → 149 + 31 = 180
- August: 31 days → 180 + 31 = 211
- September 1–20: 20 days → 211 + 20 = 231 days
You have 231 days (about 33 weeks) to plan.
Retirement and age milestones
If you were born on May 15, 1970, when do you turn 65?
- May 15, 1970 + 65 years = May 15, 2035
- From today (February 1, 2026) to May 15, 2035:
- That's 9 years, 3 months, and 14 days away
- Approximately 3,382 days from now
Use this for retirement planning, Social Security eligibility, and milestone birthday countdowns.
Date Quick Reference
Common time intervals with their conversions and typical uses:
| Interval | Days | Weeks | Months (approx) | Common Use |
|---|---|---|---|---|
| 1 week | 7 | 1 | 0.23 | Weekly meetings, recurring tasks |
| 2 weeks | 14 | 2 | 0.46 | Biweekly pay periods |
| 1 month | 30–31 | 4.3 | 1 | Monthly bills, rent |
| 30 days | 30 | 4.3 | 0.99 | Trial periods, short-term loans |
| 60 days | 60 | 8.6 | 1.97 | Return policies, payment terms |
| 90 days | 90 | 12.9 | 2.96 | Quarterly reports, project milestones |
| 6 months | 182–184 | 26 | 6 | Half-year reviews, seasonal planning |
| 180 days | 180 | 25.7 | 5.92 | Warranty periods, visa stays |
| 1 year | 365 (366 leap) | 52.1 | 12 | Annual contracts, subscriptions |
| 1,000 days | 1,000 | 142.9 | 32.9 | Long-term goals, anniversaries |
| 10,000 days | 10,000 | 1,428.6 | 328.8 | Major life milestones (about 27.4 years) |
Use this table for quick estimates. For exact calculations, always use the calculator above — month lengths vary and leap years affect the totals.
Frequently Asked Questions
Subtract the earlier date from the later date. For example, January 1 to March 1 in a non-leap year: 31 (Jan) + 28 (Feb) = 59 days. This calculator does it automatically — just enter both dates.
By default, it calculates the difference (exclusive of the start date). To include both endpoints, add 1 to the result. Monday to Friday is 4 days difference, but 5 days inclusive.
Leap years add February 29 every 4 years (except centuries not divisible by 400). If your date range crosses Feb 29 of a leap year, the calculator automatically includes that extra day.
Business days exclude weekends. Estimate: total days × 5/7 ≈ business days. For 30 calendar days, that's roughly 21–22 business days. Exact counts depend on which weekdays your range spans.
Start with your base date and count forward, accounting for month lengths. For 90 days from January 15: 16 (rest of Jan) + 28 (Feb) + 31 (Mar) = 75 days to March 31, then 15 more = April 15.
Enter today's date in the calculator and add 100 days. The result shows the exact future date and day of the week. Useful for project deadlines, countdown events, and planning milestones.
Use the calculator with your birth date as start and today as end. For example, someone born July 4, 1990 is approximately 12,965 days old as of January 1, 2026.
A standard year has 52 weeks and 1 day (365 ÷ 7 = 52.14). A leap year has 52 weeks and 2 days. This is why the same date falls on a different day of the week each year.