Date Calculator: Days Between, Adding/Subtracting Time, and Date Math
Date math sounds simple but trips people up — leap years, varying month lengths, time zones, and the question of whether the start date counts as day zero or day one. This calculator handles all the edge cases so you don't have to.
The most common date calculations are: how many days are between two dates, what date will it be N days from now, and what's the difference in years/months/days between two dates? All three involve the same calendar — but the answers can differ based on convention.
EllyTools handles leap years correctly (every year divisible by 4 except century years not divisible by 400), respects month lengths, and computes between-date differences exclusively (so Day 1 to Day 1 = 0 days, Day 1 to Day 2 = 1 day).
Three modes of calculation
**Difference between two dates**: input two dates, get the gap in days (and optionally years/months/days breakdown). Useful for anniversaries, due dates, project tracking.
**Add/subtract days from a date**: input a starting date and a number of days, get the resulting date. Useful for shipping estimates, contract dates, deadlines.
**Add/subtract years/months**: same idea but in larger units. Adding 6 months to January 31 gives July 31; adding 1 month to January 31 gives February 28 (or 29 in a leap year) — the calculator handles month-length edge cases.
Common pitfalls
- •Counting the start day as day 1 when the convention is exclusive. For 'in 7 days' starting Monday, the answer is the following Monday (7 days later), not Sunday (6 days later).
- •Leap year confusion. 2024 is a leap year (Feb 29 exists); 2100 will NOT be (century years skip leap year unless divisible by 400). 2000 was a leap year, 2400 will be.
- •Time zone effects. Two dates entered in different time zones can be 'the same day' or 'adjacent days' depending on interpretation. The calculator uses calendar dates without time-of-day.
- •Adding months to end-of-month dates. January 31 + 1 month is naturally interpreted as 'last day of February.' Different software does this differently.
Extended FAQ
Is the start date day 0 or day 1?
EllyTools' difference calculation is exclusive — Monday to Monday is 0 days; Monday to Tuesday is 1 day. This matches how most contracts and shipping estimates count.
Does it handle dates before 1900?
Yes, using the proleptic Gregorian calendar (extending today's calendar backward, even though it wasn't in use). For genuine historical dates pre-1582, consult a Julian-calendar calculator.
Why does adding 1 month sometimes give a different day-of-month?
Adding 1 month to January 31 lands on February 28 (or 29) because February has fewer days. This is the standard 'last-day-of-month' rule, but some software uses other conventions.
Are my dates stored?
No — calculations run entirely in your browser.