A question about portkeys

[suehpfan]

No: HPFGUIDX 91652

Sue ponders:

In the beginning of GoF (I do not have my book :() Mr. Weasley 
explains that a portkey is used to transport people at a particular 
time to another place.  As a matter of fact there is a great deal of 
discussion about the time on Stoteshead Hill, they even counted 
down.  After the QWC, the whole group went to try to get an "early 
portkey".  How is it then, that the portkey in the maze took Cedric 
and Harry the moment they touched it?

Any answers out there?
Sue