Example: 25 April 1990
The digits give 1+9+9+0+0+4+2+5=30. Next, 3+0=3. The result is 3. Leading zeroes in a month or day do not change the sum. Convert dates from another calendar to the Gregorian calendar first.
When to keep 11, 22 and 33
For 8 January 2000, the full sum is 2+0+0+0+0+1+0+8=11, so stop there. For 1 January 1980, the sum is 20, which reduces to 2. For 3 January 1980, the sum is 22, which is retained. Do not stop at a temporary total of 11 before adding the remaining birthday digits.
What happens to 10, 44 and 0?
In this method, 10 reduces to 1 and 44 reduces to 8. They are intermediate sums, not additional final numbers. A valid accepted birthday cannot produce zero. Continue reducing a larger sum by the same rule.
Why calculators can disagree
Some methods reduce the year, month and day separately before adding them. This calculator adds all the original digits first. For 9 November 2000, that produces 13, then 4. A separate-component method that keeps the month as 11 can produce 2+11+9=22. Agree on a method before comparing results.
Accepted dates
The calculator accepts real Gregorian dates from year 1 through today. It checks leap days. You do not need a birth time or a name.