clue four: what’s with the dates?

 There are two years to find on this mural.  One is the date the ship set sail while the other is the date the mural was created.  Using these dates, you’ll be able to shift the coded letters in alphabetical sequence.  “+” means forward in the alphabet (e.g., A, B, C, D…) and “-” means backward in the alphabet (e.g., C, B, A, Z, Y…).