How Age and Date Calculations Actually Work

Exact age in years is not simply the difference between the current year and the birth year. You must also check whether this year's birthday has already passed. If you were born on December 15, 1990, and today is July 28, 2025, your birthday has not occurred yet this year — so your age is 34, not 35. Age in months and days requires accounting for the different lengths of calendar months and leap years.

A solar year is approximately 365.2422 days long. The Gregorian calendar compensates for the extra ~0.2422 days by adding a leap day (February 29) every four years. However, to stay accurate, century years (1700, 1800, 1900) are not leap years — unless they are also divisible by 400 (2000 was a leap year; 1900 was not). This means there are 97 leap years in every 400-year cycle.

People born on February 29 (leap day) technically have a birthday only once every four years. Legally and culturally, most countries treat March 1 (or February 28) as the "official" birthday in non-leap years, but this varies by jurisdiction. Date difference calculators must account for leap years to return an accurate number of days between two dates.

South Korea traditionally used a unique age-counting system where a person is considered 1 year old at birth and gains a year every January 1st, rather than on their actual birthday. Under this system, a baby born on December 31 would be 2 years old the very next day (January 1). This "Korean age" (세는나이) typically makes a person 1–2 years older than their international age.

South Korea officially switched to the international age system (만나이) in June 2023 for most legal and administrative purposes. However, the traditional system is still sometimes used colloquially. Our age calculator uses the international standard (years since birth date), which is now the official standard in Korea as well.

Knowing the exact number of days between two dates has many practical applications: calculating project deadlines, determining how many days until an event, figuring out if a warranty or contract is still valid, calculating interest on loans (which often accrues daily), and determining eligibility ages for programs with specific age requirements. Our date difference calculator handles all of this automatically, accounting for leap years and varying month lengths.