Calculating someone's age seems trivial until you encounter birthdays that haven't occurred yet in the current year, or month boundaries with varying lengths (28, 29, 30, and 31 days). Here is the verified method to calculate exact age by hand.
The 3-Step Manual Calculation Formula
Step 1: Determine Completed Years
Subtract the birth year from the target year. If the target month/day is earlier than the birth month/day, subtract 1 full year from the total.
Step 2: Determine Completed Months
From the most recent birthday anniversary, count full whole calendar months forward up to the target month.
Step 3: Count Remaining Days
Count the days between the monthly milestone and the target date, taking into account the exact days in the preceding month.
Worked Example
Suppose Date of Birth = October 25, 1995, and Target Date = August 31, 2026:
• Completed Years: 30 Years (Oct 25, 1995 to Oct 25, 2025)
• Completed Months: 10 Months (Oct 25, 2025 to Aug 25, 2026)
• Remaining Days: 6 Days (Aug 25 to Aug 31, 2026)
➔ Total Age = 30 Years, 10 Months, 6 Days