Take someone born June 15, 1990, and calculate their age as of January 1, 2027. You can't just subtract 15 from 1 and 6 from 1 — both come out negative. The calculator handles this the same way you'd count on your fingers: borrow a month's worth of days when the day count goes negative, then borrow a year's worth of months when the month count goes negative. Worked through fully, that birth date and reference date give an age of 36 years, 6 months, 17 days — 13,349 total days lived, or 438 total months, with 165 days left until the next birthday.
Total days lived is a fun number to check against a few common milestones — turning 10,000 days old happens at almost exactly 27.4 years, and 20,000 days lands around 54.8 years. Change "Calculate age as of" to a birthday and check the "Total days lived" figure against these — it's a genuinely different way to think about a birthday than the year count alone, and the exact day it happens is easy to miss unless you're specifically counting toward it.
It's the exact number of calendar days between your date of birth and the reference date, accounting for leap years automatically.
Yes — change "Calculate age as of" to any date, past or future, to see their age at that point in time.
Age in months and days depends on how many days are in the specific months you're counting through — the calculator handles this correctly even though it can look surprising for certain date pairs.