Below is a proposed solution to the Challenge 68 – Need a Data Validation Formula
=IFERROR(IF(ISNUMBER(SEARCH("/",A2)),IF(–RIGHT(A2,4)=2018,AND(–LEFT(A2,2)>=1,
–LEFT(A2,2)<=12),IF(AND(–RIGHT(A2,4)<=2021,
–RIGHT(A2,4)>=2019),AND(–LEFT(A2,2)>=1,–LEFT(A2,2)<=4)))),FALSE)