Date Calculator: Days Between Dates and Date Arithmetic
From project deadlines to pregnancy due dates, calculating dates is a practical skill everyone needs. This guide covers date arithmetic for every common scenario.
Use our Date Calculator for instant calculations.
Calculating Days Between Two Dates
The Simple Approach
For dates in the same month:
- Just subtract: End Date - Start Date
Across Months
For dates across months, you need to count days in each month.
Example: March 20 to May 5Including or Excluding Endpoints
Exclusive (default): Days between, not counting either endpoint- Jan 1 to Jan 5 = 4 days
- Jan 1 to Jan 5 = 5 days
Days in Each Month
| Month | Days | Memory Aid |
|---|---|---|
| January | 31 | "30 days has September..." |
| February | 28/29 | Leap year = 29 |
| March | 31 | |
| April | 30 | |
| May | 31 | |
| June | 30 | |
| July | 31 | |
| August | 31 | |
| September | 30 | |
| October | 31 | |
| November | 30 | |
| December | 31 |
Rhyme to Remember
"Thirty days has September, April, June, and November. All the rest have thirty-one, Except February alone, Which has twenty-eight days clear, And twenty-nine in each leap year."
Leap Year Rules
A year is a leap year if:
- 2024: Leap year (÷4)
- 2100: NOT a leap year (÷100 but not ÷400)
- 2000: Leap year (÷400)
- 1900: NOT a leap year (÷100 but not ÷400)
Adding/Subtracting Days
Adding Days to a Date
Example: Add 90 days to March 15, 2025Subtracting Days from a Date
Example: Subtract 45 days from April 20, 2025Common Date Calculations
Weeks Between Dates
``
Weeks = Days ÷ 7
`
Months Between Dates
Approximately:
`
Months ≈ Days ÷ 30.44
`
For exact months, count calendar months.
Years Between Dates
`
Years ≈ Days ÷ 365.25
`
Or count full years plus remaining days.
Practical Examples
Example 1: Pregnancy Due Date
Standard calculation: 280 days from last menstrual period (LMP)
LMP: January 1, 2025
Due Date: January 1 + 280 days = October 8, 2025
Example 2: 90-Day Notice Period
Starting work: March 1, 2025
90-day probation ends: March 1 + 90 days = May 30, 2025
Example 3: Age in Days
Birthday: June 15, 1990
Today: January 7, 2025
Days alive: ~12,630 days
Example 4: Countdown to Event
Wedding date: September 20, 2025
Today: January 7, 2025
Days until: ~256 days
Excel/Sheets Date Formulas
Days Between Dates
`excel
=DATEDIF(A1, B1, "d")
`
Or simply:
`excel
=B1-A1
`
Add Days to Date
`excel
=A1+90
`
Date Components
`excel
=YEAR(A1)
=MONTH(A1)
=DAY(A1)
`
Workdays Only (Excluding Weekends)
`excel
=NETWORKDAYS(A1, B1)
`
Add Workdays
`excel
=WORKDAY(A1, 30)
``
Business Days vs Calendar Days
Calendar Days
All days counted, including weekends and holidays.
Business Days
Only Monday-Friday, excluding holidays.
Example: 30 calendar days ≈ 22 business days (roughly)For contracts and deadlines, clarify which is meant.
Time Zone Considerations
When calculating across time zones:
- Specify a reference time zone
- Convert dates if spanning midnight
- Be clear about "start of day" vs "end of day"
Frequently Asked Questions
Should I count the start date?
Typically no. "Days between" excludes endpoints. But clarify for your specific need.
How do I calculate age?
Age = (Today - Birthday) ÷ 365.25
Or use our Age Calculator for precise age in years, months, and days.
What about daylight saving time?
For day-level calculations, DST doesn't affect the result. For hour-level precision, account for the spring-forward/fall-back hour change.
How do I find what day of the week a date falls on?
Use the calendar function in any spreadsheet, or our date calculator. The Doomsday algorithm can calculate this mentally with practice.
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