How age on a chosen date is worked out
Age in whole years is simply the number of birthdays that have already happened by the target date. The calculator subtracts the birth year from the target year and then takes one off if the birthday has not arrived yet that year. It repeats the same idea at a finer scale for the months and days: it counts complete calendar months since the last birthday, and if the day of the month has not been reached, that month does not count. What is left over, measured from the month anchor to the target date, is the number of days.
Because real month lengths are used, the answer behaves correctly at awkward boundaries. Someone born on January 31 is exactly one month old on February 28 in a common year, and one month and one day old on March 1. This is the same convention used by pediatric growth charts, school enrollment cutoffs, Social Security and most government forms, so the figure you get here is the one those offices expect.
Total days, weeks and the next birthday
Total days is a straight count of midnights between the two dates, so the day of birth itself is not counted. On a first birthday a child has lived 365 days, or 366 if a February 29 fell inside that year. Weeks are the total divided by seven, which is how newborn ages are usually quoted for the first few months: a baby born on April 1 is 8 weeks and 3 days old on May 30. The hours line is the day count multiplied by 24 and ignores daylight saving shifts, which add or remove an hour twice a year.
The next-birthday line looks forward from the target date rather than from today, so if you ask about a date three years out it tells you the birthday that follows it. For leap-day births the tool uses February 28 in years without a February 29; most US jurisdictions treat the legal birthday as March 1 in those years, a difference of exactly one day.
When you need an age as of a date
Forms ask for age as of a fixed date more often than people expect. Kindergarten registration usually requires a child to turn five by September 1. Youth sports leagues set age brackets as of August 31 or December 31. Medicare eligibility, retirement plan catch-up contributions, and insurance rate bands all key off an age on a specific day. Court filings, custody schedules and visa applications frequently ask how old a person was on the date of an event. Set the target date to the cutoff in question and read the exact years, months and days rather than estimating.