Charging days run from midnight to midnight. So if you were to drive within the LEZ between 23:30 and 01:00 the next day, you'd need to pay for two days.
Vehicles parked in the zone but not driving are not subject to the LEZ requirements for that day, and it is administered by number plate recognition cameras. The charge is £100 per day for older vans and pick-ups, or £200 for larger commercial vehicles.
Vehicle subject to the charge are pick-ups and commercial vehicles with a 1.205 tonne unladen weight that were registered before 1 January 2002, and coaches and lorries registered before 1 October 2006.
Remember, the LEZ isn't the same as the central London congestion charge zone. The Congestion Charge applies 07:00-18:00 Monday to Friday, excluding public and Bank Holidays. If you drive within the charging zone during these times you will have to pay the Congestion Charge, even if you meet the LEZ emissions standards and have paid the daily LEZ charge.
By following this link you can pay the daily LEZ charge, pay a penalty Charge Notice (PCN) and check if your vehicles are affected and more.