that is the most assinine way to assess a law, Where I live - the hours are defined as - 1 hour before sunset and one after after sunrise, it makes this a binary system - either you are in or out. This information is already in the highway code book which the driver of the vehicle obviously knows about.
to quantify this and use in a court of law - you take that days sunrise time or sunset time (fajr n maghrib) and then compare the chalaan time - simple as that. -
Giving such "loose ends" to be decided by a patrol officer is invoking corruption. - What if the officer says - "meh kehta hoon yeh din hai - while its 2AM on the motorway"