Working
The calculator measures from the earlier date of birth to the later one: full calendar years first, then full months, then the remaining days.
13 Apr 1953 → 13 Apr 1977 = 24 years
13 Apr 1977 → 13 Dec 1977 = 8 months
13 Dec 1977 → 21 Dec 1977 = 8 days
Age gap = 24 years, 8 months and 8 days = 9,018 total days.