Not sure what you mean. You get an error or the wrong result? In what circumstances exactly, i.e. which values in which cells?

One possibility....

=SUM(IF(A3:A29="Shellie",IF(ISNUMBER(C3:C29+E3:E29),MOD(E3:E29-C3:C20,1))))*24

This is an "array formula" which needs to be confirmed with CTRL+SHIFT+ENTER