From FreeSpace Wiki
is-event-true-delay Mission Event True (Boolean operator) Returns true N seconds after the specified event in the this mission is true (or succeeded). It returns false otherwise. Returns a boolean value. Takes 2 arguments... 1: Name of the event in the mission. 2: Number of seconds to delay before returning true.
There are serious issues with this SEXP and directive events, and as such it should be avoided for that use whenever possible. Using it will in many cases prevent the directive from appearing on the HUD until it is either completed or has failed.