Under the most common convention used in Western countries, a person's age begins at zero on the day they are born and advances by one year on each subsequent birthday. A child who has been alive for three years and eleven months, for instance, is still described as three years old until their fourth birthday arrives. This is the approach used by the calculator above.
To find the precise difference between two dates, the tool first counts complete calendar years, then complete remaining months, and finally leftover days. The result tells you both the familiar "X years, Y months, Z days" breakdown and the equivalent totals expressed in larger or smaller units such as weeks, hours, minutes, and seconds.
Not every society counts age the same way. In several East Asian traditions, including the historical Chinese and Korean systems, a baby is already considered one year old at the moment of birth. Rather than advancing on individual birthdays, everyone's age then increases together on the Lunar New Year. Under this method, a child born in late December could be called two years old just a few weeks later, even though they would be barely a month old by Western reckoning.
South Korea officially transitioned to the international age-counting standard in June 2023 for legal and administrative purposes, although the traditional system still appears in casual conversation. Other countries in the region have similarly mixed practices, using the Western system for official documents while retaining the traditional count in everyday life. If you need to compare ages across these systems, keep in mind that the traditional count can be one or even two years higher than the international one.
Calculating the gap between dates that fall near the end of a month requires some careful handling. The span from January 28 to February 28 is clearly one month, but what about January 31 to February 28? Because February has no 31st day, this tool treats that interval as one full month: it considers February 28 to be the end of its month, just as January 31 is the end of its month.
The same logic applies in other situations where months have different lengths. Moving from March 31 to April 30, for example, is counted as one month. This "last day to last day" approach avoids confusing negative-day results and gives the most intuitive answer for everyday age calculations.
