Date Difference Calculations: Why "How Many Days Between" Is Harder Than It Seems

"How many days between January 1 and March 24?" Go ahead, count it in your head. Most people say 82 or 83 depending on how they're thinking about it. The correct answer is 83 — but ask ten developers to write the code, and at least three will get it wrong on the first try. Date difference calculations are one of those things that seem trivial until you actually dig into the edge cases. The Off-By-One Trap Let's start with the most common mistake: const start = new Date ( ' 2026-01-01 ' ); const end = new Date ( ' 2026-03-24 ' ); const diffMs = end - start ; const diffDays = diffMs / ( 1000 * 60 * 60 * 24 ); console . log ( diffDays ); // 82 Is it 82 or 83? It depends on what you're actually asking: Exclusive end (don't count the end date): 82 days Inclusive end (count both the start and end date): 83 days There's no universally correct answer — it depends on your context. Booking systems typically use exclusive end dates (if you check in Jan 1 and check out Jan 3, your stay is 2 nights