I think it is fair to say that the French have a fairly ambivalent attitude towards time. Anyone who has held an office job here will know that a meeting scheduled for 9am rarely starts before 9:15, and any appointments with doctors and dentists are usually booked on the basis of what the medical secretary feels will work best in the professional’s diary. This is seldom what does work best, (funnily enough appointments can actually take a full 20 minutes or more) so you get used to expecting a long wait if you arrive for an appointment on time. While this can be infuriating if you are not used to it, the French seem to find this “normal”.
It is interesting that this notion of approximation carries over into the French language for certain expressions involving time. For example, the expression for a fortnight – as in two weeks, fourteen days – is in fact “quinze jours” or “une quinzaine”, meaning fifteen days. When it comes to telling someone about what happened last night the French will say “cette nuit” literally meaning this night, but when talking about what they are going to do tonight they also say “ce soir” – this evening.
However, in some instances the French are actually more precise in their use of expressions for time than you might expect. One example of this, (in my case, learnt the hard way), involves the use of the work “prochain” meaning “next”. In English, an appointment made for “next Thursday” means the Thursday of the following week. Here French speakers are actually more accurate than Anglophones because “jeudi prochain” literally means the next Thursday to come, which may be in the same week or even tomorrow. The same is true for the use of “dernier” meaning “last”, so “mardi dernier” means the last Tuesday that occurred, and not Tuesday of last week, as in English.
The use of “en” or “dans” is also a precise science when used to express time in French. “En une semaine” and “dans une semaine” do not mean exactly the same thing, even if both may be translated by “in a week”. “En une semaine” means it will take a week to do something, the duration will be one week, whereas “dans une semaine” means something will start in one week’s time.
I still haven’t found a satisfactory answer as to why delays are generally given at “dix jours” (rather than one or two weeks) for most purchases. I can only assume that this is to give suppliers more time to complete their orders, whilst still appearing to be a low-ish count in terms of the number of days required.
There are many French expressions involving time, one of my favourites is “Ce n’est pas demain la veille” which literally means “tomorrow is not the day before [something]”, a rather elaborate way of saying that something won’t be happening soon. For instance, you might say that when it comes to understanding timekeeping in France “ce n’est pas demain la veille”.