How to Calculate the Number of Days Between Two Dates

The Manual Method

For simple calculations within the same year, count the remaining days in the start month, add full months in between, then add the days in the final month. But this approach becomes error-prone quickly when months of different lengths and leap years are involved.

The Julian Day Number Method

The most reliable approach converts each date to a Julian Day Number (JDN) — a continuous count of days since a fixed reference date — then subtracts. This automatically handles all month-length variations and leap years.

A simplified version for modern dates (after 1 March 1900):

1. y = year, m = month, d = day
2. If month ≤ 2: adjust y by −1, m by +12
3. A = floor(y/100), B = 2 − A + floor(A/4)
4. JDN = floor(365.25(y+4716)) + floor(30.6001(m+1)) + d + B − 1524.5
5. Days difference = JDN(date2) − JDN(date1)

Worked Examples

Business Days vs Calendar Days

Many deadlines are measured in business (working) days rather than calendar days. Business days exclude weekends (Saturday and Sunday) and public holidays. A 30-calendar-day period contains approximately 21–22 business days depending on when it falls in the week.

Practical Use Cases

• Contract deadlines: "30 days from signing" — add 30 days to the signature date
• Pregnancy due date: Add 280 days (40 weeks) to the first day of last menstrual period
• Countdowns: Days until a holiday, event, or retirement date
• Age in days: See our article on calculating exact age in days, weeks, and months