"03/05/2024" means March 5th in the US — and May 3rd almost everywhere else. That single formatting convention causes more real-world date-diff mistakes than leap years and time zones combined, because the input looks perfectly valid either way you read it.
MM/DD/YYYY vs. the rest of the world
The US is one of the few countries that puts the month before the day. Most of Europe, Latin America, and Asia write DD/MM/YYYY instead, and a growing number of systems use the unambiguous ISO 8601 order, YYYY-MM-DD. A date like 04/07/2024 is April 7th to an American reader and July 4th to nearly everyone else — and a date-diff calculation built on the wrong assumption won't throw an error, it'll just be quietly wrong by weeks or months.
Leap years
Every four years, February gets a 29th day, except for years divisible by 100 that aren't also divisible by 400 — which is why 2000 was a leap year but 1900 wasn't. Skip this rule and a day count spanning a different number of leap years comes out wrong.
Time zones and day boundaries
When dates carry a time of day rather than just a calendar date, a time zone difference can shift the day count by one: 11:30pm in New York and 8:30pm the same moment in Los Angeles can fall on different calendar dates depending on which zone you're counting in.
Why you'd need this
- Calculating the exact number of days until a deadline or since a project's start date.
- Working out the length of a lease, subscription, or warranty period.
- Catching a date parsed with the wrong locale before it corrupts a report.
Why "months between two dates" doesn't have one right answer
Subtract a month from March 31st and the result is ambiguous — February 31st doesn't exist, so some systems round down to February 28th while others roll the extra days into March 3rd. Both are defensible, but they disagree, which is worth remembering when a date-diff result doesn't match another tool's.