This should not return any errors.
Code:
SecOnes var byte
Seconds var byte
SecOnes = Seconds & $0f
Of course the result will be unknown since none of the variables are initialized, but it
definitely should not return an error.

Why does it say $0f ???
Because a value of $0F (%00001111) ANDed with another 8-bit value will return all
0's in the upper 4-bits of the result MASKing the upper 4-bits out.