you have to subscribe to OnAfterCopyJob publisher of Copy Job codeunit like below
[EventSubscriber(ObjectType::Codeunit, Codeunit::"Copy Job", 'OnAfterCopyJob', '', false, false)]
local procedure OnAfterCopyJob(var TargetJob: Record Job; SourceJob: Record Job);
begin
TargetJob."Starting Date" := 0D;
end;