Date & Time
5 min read
•Updated 2026-09-18
How Date and Time Differences Are Calculated: Chronological Math & Workdays
An exploration of Gregorian calendar arithmetic, UTC epochs, leap second handling, and working day calculation algorithms.
By AnyCalcHub Engineering Desk
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1. The Millisecond Difference Algorithm
Dates in computing are measured as milliseconds elapsed since the Unix Epoch (January 1, 1970 UTC). To find elapsed days between two dates, compute the absolute difference in milliseconds and divide by 86,400,000.
Mathematical Formula
Days = Math.round(|Timestamp 2 - Timestamp 1| / (1000 × 60 × 60 × 24))
Step-by-Step Worked Example
Start Date2026-01-01
End Date2026-03-31
Step 1:January has 31 days
Step 2:February (non-leap) has 28 days
Step 3:March has 31 days
Step 4:Sum: 31 + 28 + 31 = 90 days
Final Result: 90 Calendar Days Difference
Frequently Asked Questions
Common questions and mathematical nuances for this calculation.
How do you accurately count working business days excluding weekends?
Iterate through each calendar day from start to end date. If the day of the week is neither Saturday (6) nor Sunday (0), increment the business day counter.
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