Whenever you touch anything under control / under separation rules / subject to slotting in Europe 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 one delay in the system, which you frequently have to do leaving late, but there is usually no 'leave earlier' in the system and 'bring forward' is known to crash certain routines in European airflow (as does delaying multiple times) - the system in Europe is authority based, not customer centric. Btw, in busy airspace, such as Brussels, the slot may often be only +15 based on offblock time, or even more restrictive with the system going down all the way to +5 -> read the AIP comments for that, if you don't want to check with tower.
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 ...). If going across borders, not all flightplans are passed ahead of schedule to the taking ATC or FIS, so leaving early would get you into entering foreign airspace on no flightplan (not relevant for countries not passing VFR flightplans from AIS to ATC or FIS anyways - not so uncommon).
Last edited by ChickenHouse; 30th Dec 2017 at 00:28.