Date quirks

Anyone born in ’22 has their 99th birthday this year; born in ’33, 88th; born in ’44, 77th; born in ’55, 66th; born in ’66, 55th, etc

All of the above are multiples of 11, and the sum of the repeated digits is also 11 – 2 + 9, 3 + 8, 4+ 7, 5 + 6, etc.

The other quirk is that anyone born in ’29 has their 92nd birthday; born in ’38, 83rd, born in ’47, 74th, born in 56, 65th, etc.

This time the sum of the actual digits is 11.

Can anyone explain why this works so wonderfully?

Peter Gordon, Ilam