Count anniversaries first
An age in years describes completed yearly anniversaries. The year of a birth date alone is not enough: the month and day determine whether the next anniversary has arrived. A person born on October 9, 2000 has not reached their October anniversary on October 8, 2026.
Keep calendar units intact
Months have different lengths. Dividing a number of days by 30 does not reliably give a calendar-month count. AgeUse advances by complete months from the original birth date, then counts the remaining days from the last completed month.
Use the right result for the question
For a calendar age, use the years, months and days display. For the size of a date interval, use total days. Both are useful, but they answer slightly different questions. Keep the chosen reference date with the result whenever you write it down.