The good news--only in Europe do you have to put this nonsense. In fact, one can file a flight plan for 00Z, be ready at 2330Z and be forced to wait til 00Z WITHOUT any slot regulation being applied. Today, in the NE, between NYC and BOS, filed flight plan, went downstairs and got a clearance.
I was always taught to plan on being at the hold point 5 minutes before the slot time and that the slot time is the PILOT's time, the window of -5 and +10 is for ATC.
GF