The concept of decimal time, wherein a day is broken down into multiples of 10.
By the eve of the French Revolution, the idea had evolved into a year split into 12 months of 30 days apiece.
That there are 365 days in a year is an immutable fact dictated by the movement of the Earth around our local star. So, 12 months of 30 days apiece resulted in 5 days (6 in a leap year!) left over. These, the revolutionaries reserved for national holidays.
Each week was divided into 10 days, every day was split into 10 equal hours, those were split into 100 minutes, with each minute divided into 100 seconds (roughly 1.5 times longer than conventional minutes) and each second into 1000 “tierces.” Individual tierces could also be divided into 1000 even tinier units, called “quatierces.”
Decimal time was formally adopted by National Convention decree in 1793, “The day, from midnight to midnight, is divided into ten parts, each part into ten others, and so forth until the smallest measurable portion of duration.” As such, midnight would be denoted as 00:00 while noon would be 5:00.