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)