String manipulation with conditionals

Hi dears, i need to arrange the date strings to YYYY from different formats (YY, YYMMDD up to 1996 and DDMMYYYY for upper).

I´m trying to create the follow:

if the length is 6 or less, extract the firsts 2 characters and merge to “19”, else extract the right 4 characters.

I suppose it is super ease because the excel possibilities. But because I´m a ultra beginner, any help is welcome.