How to Calculate Days Between Dates Without Off-by-One Errors
Separate elapsed time from inclusive calendar counting when measuring date ranges.
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Elapsed days and inclusive days differ
The elapsed difference from Monday to Tuesday is one day. A schedule that counts both Monday and Tuesday as active dates may describe the same range as two calendar days.
State which interpretation you need before using a result in billing, deadlines or reports.
Calendar months are not equal lengths
A month can contain 28, 29, 30 or 31 days. That is why a year-month-day breakdown is different from dividing total days by a fixed number.
Use total days for exact elapsed duration and calendar units when the human calendar relationship matters.