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5 min read
May 25, 2025

How to Calculate Days Between Dates: Common Questions Answered

Date calculations seem simple until you hit edge cases: leap years, month-end overflow, timezones.

How to Calculate Days Between Dates: Common Questions Answered

Calendar days: Every day counts. Simply subtract dates.

Business days: Monday–Friday only. Approximate: calendar days × 0.714

For precise counts (especially across month boundaries), our Days Calculator counts day by day.

Leap Year Rules

  • Divisible by 4 → leap year
  • Divisible by 100 → NOT leap year (except...)
  • Divisible by 400 → IS leap year

    • 2024 is a leap year; 2100 will not be. "90 days from February 1, 2024" crosses February 29 and lands on May 1 — in a non-leap year it's May 2.

    Month-End Edge Cases

    "One month from January 31" — February has no 31st day. Systems handle this differently:

  • Snap to last day: February 28
  • Overflow to March: March 3

    • Always specify a calendar date in contracts rather than a duration.

    ISO 8601: Always Use YYYY-MM-DD

  • 2025-06-03 ✓ (unambiguous globally)
  • 03/06/2025 ✗ (June 3 in US, March 6 in Europe)

    • Timezone Warning

    "End of business June 30" in the UK = July 1 at 00:00 PKT (UTC+5). Always confirm the timezone for critical deadlines.

    Written by the GMC Tools team

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