How to Calculate Days Between Two Dates (Date Math Explained)

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

1. January 20 to January 31 = 11 days
2. All of February = 28 days (or 29 in a leap year)
3. March 1 to March 10 = 10 days
4. Total: 11 + 28 + 10 = 49 days

Days in Each Month

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:

1. Divisible by 4 → leap year
2. BUT divisible by 100 → NOT a leap year
3. BUT divisible by 400 → leap year

Common Date Calculations

How Many Days Until...

Weeks and Months Conversion

Calculating Age

Age = Today's Date − Date of Birth

But age isn't just about days — we count complete years:

1. Subtract the birth year from the current year
2. 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)

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?

1. March: 31 − 1 = 30 remaining days
2. April: 30 days (running total: 60)
3. May: 30 more days needed → May 30
4. Answer: May 30 (90 days from March 1)

Subtracting Days from a Date

What date was 45 days before June 15?

1. June 1-15 = 15 days
2. All of May = 31 days (running total: 46)
3. Need 45 days → May 1 = 46 days back, so May 2
4. 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

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