How to format 2 number yy values to 4 number yyy values in datetimes (19XX - 20XX)?

Hi Business Engineers,

I am formatting date time with 2 numbers for years (yy) to 4 numbers (yyyy). When all the date times are of the 20th century, i add 1900 years, and I get a correct 4 numbers yyyy.

But when the original date times are also from 21ths century ('17) how should I divide to add 1900 years or 2000 years in orde to complete '70 to 1970 or '17 to 2017?