In reply to Solaris: Agreed, that is the accepted thinking and will be expounded by all carnet holders because, in general, Alpinism is dictated by early starts and early finishes, by moving fast and light, and by getting to the summit and off before the sun has had time to do its damage. However this is not always the case. Depending on where you are, what time of the year it is, and whats been happening met-wise, the conditions underfoot and on slope can be just as good and safe at 4pm as they are at 8am.
My point, as in earlier reply, was always give yourself plenty of time to complete safely. So, if you find you're often outside the times quoted in a guide plan extra time in so that you are off the mountain at a time dictated by time of year, conditions, weather and your own capabilities.