You could, but then it would it depend on the date format in use not changing - so assembling the the string from the date elements is a better solution.
Replace ( YourDate ; 7 ; 2 ; "" ) ;
"/" ; "-" )
Okay. I ended up just creating this custom function to get a 2-digit year with padded days and months, for any date d:
Right ( "0" & Day ( d ) ; 2 ) & "-" &
Right ( Year ( d ) ; 2 ) )