Originally Posted by
ChickenHouse
Whenever you touch anything under control / under separation rules in your flightplan, your slot is 'time filed' to 'time filed +30'. Leaving early is a bad idea if eurocontrol is not expecting you in certain airspace for separation so soon. It is no problem to file delays in the system, which you frequently have to do leaving later than slot, but there is no 'leave earlier' in the system.
If you start at an uncontrolled airfield, ATC won't tell you you are wrong over the frequency, but leaving from controlled tower, most of the times you won't get a clearance to leave ahead of schedule (because they would have to call and announce you being early over the phone ...).
Makes sense, so the bottom line is don’t be getting ahead of yourself when it comes to your chosen departure time.