ifElse-Formular

hello,

i already searched for an answer, but non of the given Answers worked for me. I think i just dont get it. I hope someone can help me out!

I have a "yes" and "no"- drop-down. If the user say "yes" the date of an other (date)-field should change to 01.01. of the current year. If the user says "no", the (date)-field must stay clear so the user has to type a date in it. 

Many thanks in advance!