How Date Calculations Work
Date math looks simple until you hit edge cases: months have different lengths, leap years add an extra day, and crossing month boundaries gets tricky. Here's the straightforward logic behind the calculator.
Date Difference Formula
Total Days = End Date - Start Date (in milliseconds) ÷ 86,400,000
86,400,000 = 24 hours × 60 minutes × 60 seconds × 1,000 milliseconds
Worked Example: Project Deadline
Your project starts on February 15, 2025 and is due in 90 days. When is the deadline?
- February has 28 days in 2025 (not a leap year), so 13 days remain in February
- 90 - 13 = 77 days left after February
- March has 31 days: 77 - 31 = 46 days left after March
- April has 30 days: 46 - 30 = 16 days left after April
- 16 days into May = May 16, 2025 (a Friday)
Worked Example: Pregnancy Due Date
Pregnancy is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP).
If your LMP was April 1, 2025:
- April: 29 days remaining (April 2-30)
- May: 31 days (total: 60)
- June: 30 days (total: 90)
- July: 31 days (total: 121)
- August: 31 days (total: 152)
- September: 30 days (total: 182)
- October: 31 days (total: 213)
- November: 30 days (total: 243)
- December: 31 days (total: 274)
- 6 more days into January = January 6, 2026
Calendar Days vs. Business Days
This distinction matters for contracts, shipping estimates, and legal deadlines. Here's the difference:
| 30 Calendar Days | 30 Business Days |
|---|---|
| Exactly 30 days, including weekends | About 42 calendar days (6 weeks) |
| Jan 1 → Jan 31 | Jan 1 → Feb 11 (approximately) |
Real-world example: Amazon's "5-7 business days" shipping means 7-10 calendar days, not 5-7. A package ordered on Friday won't count Saturday or Sunday, so day 1 is Monday.
Leap Year Rules
Not every year divisible by 4 is a leap year. The full rule:
- Divisible by 4? Leap year candidate
- Divisible by 100? NOT a leap year, unless...
- Divisible by 400? IS a leap year
Examples:
- 2024: Leap year (divisible by 4)
- 2100: NOT a leap year (divisible by 100, not by 400)
- 2000: WAS a leap year (divisible by 400)
Quick Reference Table
| Time Period | Days | Common Use |
|---|---|---|
| 1 week | 7 days | Sprint cycles, pay periods |
| 2 weeks | 14 days | Notice periods, bi-weekly pay |
| 30 days | 30 days | Trial periods, billing cycles |
| 90 days | ~3 months | Probation periods, quarterly reviews |
| 180 days | ~6 months | Passport validity, visa durations |
| 365 days | 1 year | Annual subscriptions, warranties |
Frequently Asked Questions
How do I calculate the number of days between two dates?
Enter both dates in the Date Difference mode above. The calculator shows the result in multiple formats: total days, weeks and days, and years/months/days. For example, January 1, 2024 to December 31, 2024 equals 365 days, or 52 weeks and 1 day, or 11 months and 30 days.
How do I add days to a date?
Switch to Add/Subtract mode, enter your starting date, select "Add," input the number (e.g., 90), choose the unit (days, weeks, months, or years), and the result appears instantly. Adding 90 days to January 1, 2025 gives you April 1, 2025.
Does this calculator account for leap years?
Yes. The calculator uses JavaScript's built-in Date object, which correctly handles leap years. February 2024 had 29 days because 2024 is divisible by 4. February 2025 has only 28 days. When you add 365 days to Feb 28, 2024, you get Feb 27, 2025 (accounting for the leap day).
How do I calculate my pregnancy due date?
Use Add/Subtract mode. Enter the first day of your last menstrual period (LMP), select "Add," enter 280, and choose "days." This gives the standard 40-week estimate. Remember: only about 5% of babies arrive on their due date. The window is typically 37-42 weeks.
How many business days are in a month?
It depends on the month and which day it starts. January 2025 has 23 business days (starts Wednesday). February 2025 has 20 business days. The annual average is about 21 business days per month, or 260-262 business days per year.
What's the difference between calendar days and business days?
Calendar days count every day including weekends. Business days only count Monday through Friday. A "10 calendar day" deadline is 10 actual days. A "10 business day" deadline is 2 full weeks (14 calendar days). When a contract says "days," it usually means calendar days unless it specifies "business days."