How to Calculate Days Between Two Dates (and Why It's Harder Than You Think)

“How many days until my vacation?” “How long have I been at this job?” “How many business days between these two project milestones?” Date arithmetic sounds simple — until you encounter months with different lengths, leap years, and the question of whether to include the start date.

Total days: the simple case

The easiest approach: convert both dates to a common reference (milliseconds since epoch) and subtract:

days = floor((date2 - date1) / 86400000)

This gives you the absolute gap in 24-hour periods. No calendar quirks — just pure elapsed time.

Calendar difference (years, months, days)

This is trickier. “From January 31 to March 1” — is that 1 month and 1 day? Or 1 month and 0 days? Different systems disagree.

The standard approach:

1. Subtract years
2. Subtract months (borrowing a year if negative)
3. Subtract days (borrowing the previous month’s length if negative)

This gives a human-readable “2 years, 3 months, 15 days” breakdown.

Business days

Most people asking “days between dates” actually want working days — excluding weekends and optionally public holidays.

Business days = Total days − Saturdays − Sundays − Holidays

Counting Saturdays/Sundays in a range: iterate through the dates and skip weekends. For public holidays, you need a country-specific calendar.

Leap year complications

February 29 exists only in leap years (divisible by 4, except centuries not divisible by 400). A range spanning Feb 28–Mar 1 is 1 day in leap years and also 1 day in non-leap years — but the total days in the full year are different (365 vs 366).

Common use cases

• Employment tenure — HR departments calculate exact service years
• Loan tenure — Banks need exact days for interest accrual
• Project management — Sprint durations, deadline countdown
• Legal — Notice periods, statute of limitations, contract durations
• Personal — Pregnancy due dates, event countdowns

Calculate any date gap with the OurDailyCalc date difference calculator — it shows years/months/days, total days, weeks, hours, and business days.

TL;DR

• Simple days: subtract timestamps, divide by 86400000
• Calendar breakdown uses month-borrowing arithmetic
• Business days exclude weekends (and optionally holidays)
• Leap years add an extra day but the formula handles it automatically