Portkey mystery? - incomplete lexicon information

[freia80]

No: HPFGUIDX 59037

I noticed something about Portkeys while re-reading GoF (aahh, it is 
so good!) and I hope you will not kill me if this has been discussed 
before.
In chapter six ("The Portkey") Arthur explains to Harry: "They're 
objects that are used to transport wizards from one spot to another 
at a prearranged time". 
Steve's Lexicon definition is: "A Portkey is an enchanted object, 
often a piece of supposedly worthless junk, which when touched will 
transport a person to a preprogrammed location". Nothing is mentioned 
about the "prearranged time" bit in this definition, and indeed, 
later in the book - when the Triwizard Cup acts as a Portkey - it 
works like that: when it is touched.
So I'm wondering if I've missed something here or if there are simply 
many different kinds of Portkeys. Thoughts?

Mrs. M