"How Old Am I, Exactly?" — And Other Date Math
Two questions people search for constantly, and the one counting mistake that trips up almost everyone doing it by hand.
Both of these come down to subtracting dates — but calendars don't divide evenly, which is exactly why doing this by hand goes wrong more often than it should.
Q: I know my birth date. Why isn't my age just this year minus my birth year?
Because that only works after your birthday has passed this year. Someone born on 15 November, checking their age on 1 August, is still one year younger than a plain year-subtraction suggests — their birthday for this year hasn't happened yet.
The correct approach subtracts years, then checks whether the current month/day has reached the birth month/day yet — borrowing a year if not, the same way you'd borrow when subtracting a larger number from a smaller one. The Age Calculator handles that borrowing automatically and also shows total days, weeks, and months lived, plus a countdown to your next birthday.
Q: I need to count the days between two dates. Do I include both dates or not?
This is the detail that causes the most off-by-one errors. If a booking runs from the 1st to the 5th of a month, is that 4 days or 5 days? Both are "correct" depending on what you're counting — 4 nights of a hotel stay, but 5 calendar days if you're counting every day the room was occupied, including both the 1st and the 5th.
The Date Difference Calculator has an explicit toggle for this — leave it off for a plain gap between two dates (like days until a deadline), or turn it on when both endpoints should count (event durations, inclusive booking ranges).
Q: Why does a date-difference tool show months differently than I'd expect?
Because months aren't a fixed number of days — going from 31 January to 28 February is one month, even though that's fewer days than going from 1 March to 28 March. A tool that divides total days by 30 to approximate "months" will drift out of sync with the actual calendar the longer the date range gets. Calculating years/months/days the way a calendar actually works (like both tools here do) avoids that drift entirely.