The Quick Answer
To find the days between two dates, subtract the earlier date from the later date.
January 15 to March 20 = 64 days
For more complex calculations (business days, months, years), use our calculator:
How Date Subtraction Works
Simple Case: Same Month
March 5 to March 22
- 22 − 5 = 17 days
Across Months
January 20 to March 10
- January 20 to January 31 = 11 days
- All of February = 28 days (or 29 in a leap year)
- March 1 to March 10 = 10 days
- Total: 11 + 28 + 10 = 49 days
Days in Each Month
| 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 |
Memory trick: "30 days has September, April, June, and November. All the rest have 31, except February."
Leap Year Rules
A year is a leap year if:
- Divisible by 4 → leap year
- BUT divisible by 100 → NOT a leap year
- BUT divisible by 400 → leap year
| Year | Leap Year? | Why |
|---|---|---|
| 2024 | Yes | Divisible by 4 |
| 2025 | No | Not divisible by 4 |
| 2100 | No | Divisible by 100, not 400 |
| 2000 | Yes | Divisible by 400 |
| 2028 | Yes | Divisible by 4 |
Common Date Calculations
Planning Vacations, Deadlines, and Business Days
Most date calculations are practical: how many days until a trip, how many business days before a deadline, or how many weeks until an event.
| Question | What to enter |
|---|---|
| How many days until vacation? | Today as the start date, departure date as the end date |
| How many business days before a deadline? | Use the business-day result, then account for holidays |
| How many weeks until an event? | Divide days by 7 or use the calculator's weeks result |
| How long between two payments? | Use calendar days unless the agreement says business days |
| How old will someone be on a date? | Birth date as the start date, target date as the end date |
If a deadline depends on business days, check whether holidays count. Calendar days and business days can produce very different answers.
How Many Days Until...
| Event | From Jan 1 |
|---|---|
| Valentine's Day (Feb 14) | 44 days |
| Tax Day (Apr 15) | 104 days |
| Independence Day (Jul 4) | 184 days |
| Halloween (Oct 31) | 303 days |
| Christmas (Dec 25) | 358 days |
| New Year's Eve (Dec 31) | 364 days |
Weeks and Months Conversion
| Days | Weeks | Months (approx) |
|---|---|---|
| 7 | 1 week | — |
| 14 | 2 weeks | 0.5 months |
| 30 | 4.3 weeks | 1 month |
| 60 | 8.6 weeks | 2 months |
| 90 | 12.9 weeks | 3 months |
| 180 | 25.7 weeks | 6 months |
| 365 | 52.1 weeks | 12 months |
Calculating Age
Age = Today's Date − Date of Birth
But age isn't just about days — we count complete years:
- Subtract the birth year from the current year
- If today's date is before the birthday this year, subtract 1
Example: Born March 15, 1990. Today is April 1, 2026.
- 2026 − 1990 = 36
- April 1 is after March 15 → birthday has passed
- Age: 36 years old
For exact age in years, months, and days, use our Age Calculator.
Business Days (Working Days)
Business days exclude weekends (Saturday and Sunday) and public holidays.
Quick Estimation
Business days ≈ Total days × 5/7
For 30 calendar days: 30 × 5/7 ≈ 21 business days
US Federal Holidays (2026)
| Holiday | Date | Day |
|---|---|---|
| New Year's Day | Jan 1 | Thursday |
| MLK Jr. Day | Jan 19 | Monday |
| Presidents' Day | Feb 16 | Monday |
| Memorial Day | May 25 | Monday |
| Independence Day | Jul 4 (observed Jul 3) | Friday |
| Labor Day | Sep 7 | Monday |
| Columbus Day | Oct 12 | Monday |
| Veterans Day | Nov 11 | Wednesday |
| Thanksgiving | Nov 26 | Thursday |
| Christmas | Dec 25 | Friday |
Average business days per month: 21-22 (252 per year)
Date Addition and Subtraction
Adding Days to a Date
What date is 90 days from March 1?
- March: 31 − 1 = 30 remaining days
- April: 30 days (running total: 60)
- May: 30 more days needed → May 30
- Answer: May 30 (90 days from March 1)
Subtracting Days from a Date
What date was 45 days before June 15?
- June 1-15 = 15 days
- All of May = 31 days (running total: 46)
- Need 45 days → May 1 = 46 days back, so May 2
- Answer: May 1 (45 days before June 15)
Useful Date Formulas
Day of the Week (Zeller's Formula)
Want to know what day of the week a date falls on? The Doomsday algorithm is a mental math shortcut, but for everyday use, just use a calculator.
Duration in Different Units
| From → To | Days | Hours | Minutes |
|---|---|---|---|
| 1 week | 7 | 168 | 10,080 |
| 1 month (avg) | 30.44 | 730.5 | 43,830 |
| 1 year | 365.25 | 8,766 | 525,960 |
| 1 decade | 3,652.5 | 87,660 | 5,259,600 |
Key Takeaways
- Days between dates = later date − earlier date
- Account for different month lengths and leap years
- Business days ≈ calendar days × 5/7 minus holidays
- Age is counted in complete years (subtract 1 if birthday hasn't passed yet)
- For precise calculations across months and years, use a date calculator