Why Two Age Calculators Gave You Slightly Different Answers
A one-day difference between two age or date calculators almost always comes from whether the start date is counted as day zero or day one — an inclusive-vs-exclusive convention difference, not a calculation error. Age in years, months, and days is far less prone to this than a raw total-day count.
Getting a slightly different number from two different date tools is common enough to be normal, and it almost always traces back to one specific convention difference rather than either tool being wrong.
Inclusive vs. exclusive day counting
"Days between March 1 and March 10" can be calculated as 9 days (exclusive of one endpoint) or 10 days (inclusive of both) — both are mathematically valid answers to slightly different questions, and tools don't universally agree on which convention to default to.
For age specifically, this rarely affects the years/months/days output, since birthdays are fixed calendar points — but it can affect a raw "total days alive" figure by exactly one day, depending on whether today is counted as a full day lived or not.
How the current-year adjustment is handled
If a birthday hasn't happened yet this year relative to the reference date, a correct calculation subtracts one from the naive year count. A tool that skips this adjustment will show someone's age as one year too high for part of the year — checking whether a calculator's age matches what the person would say themselves is a quick sanity check.