Seeing the light
With regard to your licence it is simple. There is only darkness and then the light. Nothing in between! The definition of 'night' for the purposes of flight has been fully defined (the qoute from the ANO) for all other times for the purpose of the Licence or rating IT IS day.
To fly at night you must have a Night Rating (Qualification) but to instruct at night you must have an Instructor Night Rating (Qualififcation). If you do not have the latter then you must not instruct during the night hours as defined for flight. A simple answer to a simple question; should an instruction flight not end before official night takes place when the Instructor is not appropiately rated, what? Answer; then the lesson should end and the Instructor uses the Night privilege of his pilots rating to take control and land.
Daylight hours commence 30 minutes before sunrise to 30 minutes after sunset, it is that simple. If as a suitably qualified instructor you may train during the night hours, then fine for many exercises. PFLs would be problematic of course. Many other lessons can be carried out very successfully at night including some navigation.
It is rare though that the student may log a lesson for two purposes. It should be logged as night qualification training or training toward the requirement for the PPL but not both, although if undertaken during official night then hours are entered into the night column. In most cases the student will have exceeded the minimum requirements for the PPL and therefore the additional night hours if appropiate could count toward the Night Qualification applied for later.