How the gap is measured
The difference between two birth dates is computed the same way an age is: complete years first, then complete months, then leftover days. Take April 12, 1988 and September 30, 1992. From April 12, 1988 to April 12, 1992 is four full years. From April 12 to September 12, 1992 is five more months, and from September 12 to September 30 is eighteen days, so the gap is 4 years, 5 months and 18 days. The total-days line gives the same span as a flat count, 1,632 days in this case.
An important property of this method is that the gap never changes. Two people born four years apart are four years apart forever, even though their ages in whole years briefly differ by five during the months when one has had a birthday and the other has not. That is why the tool reports the fixed gap separately from the two current ages, which is where most of the confusion about age differences comes from.
Reading the two ages
Both ages are shown as of a date you choose, which defaults to today. Set it to a date in the past to answer questions such as how old each sibling was when a family photo was taken, or set it to a future date to plan for a milestone birthday. If one person was not yet born on the date you pick, the tool says so rather than showing a negative age. The line at the bottom tells you how old the elder person was on the day the younger was born, which is the figure people usually want when they are talking about siblings or about parents and children.
Days, weeks and decimal years are given as well. Decimal years use 365.25 days to account for leap years, so a 1,632 day gap is 4.47 years. Use the whole years, months and days figure for anything official and the decimal only for quick comparisons.
Where age gaps matter
School districts use age as of a cutoff date, so a gap of a few months can put two children in different grades. Insurance premiums, retirement account catch-up contributions and Social Security claiming strategies all depend on relative ages between spouses. Youth sports place players in brackets by birth year or by age on a set date. In a few states, statutory age-of-consent laws include close-in-age provisions defined by an exact number of years and months between birth dates, so the precise figure rather than a rounded one is what counts.