First of all, many thank's for the replies.
Last time I checked, there were 3600 +/- 1 seconds in an hour, and unless maths has changed since I was at school, 0.5% of 3600 is still 18.
I agree, I learnt the same maths than you
. But I just find actually about 1second more or less per hour on my Arduino, may be I'm lucky with the resonator.
Thank's I didn't new about constant, I will modify this.
"Roll over is not a problem" : but when the mills() will rollover (50 days later), the substraction will not work correctly I think ?