From FreeSpace Wiki
Has arrived delay (Boolean operator) Becomes true <delay> seconds after the specified ship(s) have arrived into the mission Returns a boolean value; Takes 2 or more arguments: 1: Time delay in seconds (see above). Rest: Name of ship (or wing) we want to check has arrived.
Ships that are present at mission start (=their arrival cue is "true") do not count as arrived. Including a ship like that as an argument for this SEXP will always return "false."