Age and date differences: decide what you are counting
The same interval can contain three elapsed days or four included dates. An age can also differ when measured in another calendar. Sometimes apparently conflicting calculators are answering different questions.
Age is not elapsed days divided by 365
Years, months and days of age follow birthdays and calendar boundaries. Months differ in length and Gregorian years can be leap years. Dividing a day count by a fixed year length is an approximation, not an exact calendar age.
References: U.S. Naval Observatory — Introduction to Calendars
The input calendar is not necessarily the age calendar
A Hijri birth date can be converted to its corresponding day and used to calculate Gregorian age. That does not make the result a Hijri age. Decide whether you are changing how the birth date is entered or the calendar used to measure the age.
A tabular Hijri calendar is calculated and can differ from official declarations or local moon sightings. For official paperwork, use the recorded date and the receiving authority’s rules rather than silently substituting a conversion.
References: U.S. Naval Observatory — Introduction to Calendars
Inclusive counting and working days
A date difference usually counts day boundaries crossed. Counting every date participating in an activity may require both endpoints. Working days additionally depend on the applicable weekend and holidays.
- Use a written month or an explicit year/month/day format when sharing an ambiguous date.
- School-age calculations need the school’s starting date and age rule; age alone does not establish admission.
- Around daylight-saving changes, one calendar day is not always 24 elapsed hours. Keep those questions separate.
Worked example: a visit from October 10 to October 13
From the start of October 10 to the start of October 13, three days pass. The list October 10, 11, 12 and 13 contains four dates. A nights-based booking may use the former; an inclusive attendance record may use the latter.
Someone born May 15, 2000 has completed 25 years on May 14, 2026, and turns 26 the next day. Subtracting the year numbers alone gets ahead of the birthday.