Funny question because, intuitively, the darkness should be enough to tell.
But, in France, there is a “rule.”
There is a day and a night “official” time.
There is a day and a night “official” time.
It’s day as long as the street lighting is not on. When the street lamps go on… then it’s night.
It’s important to know if you’re driving as it’s the rule you have to observe to switch you car lights on.
It’s also a rule of thumb to stop saying Bonjour and start saying Bonsoir 😉
But we’ve seen in the K.I.S.S. Method that you can keep saying Bonjour at any time… much simpler 😉