It's even more sophisticated than that. Each hour of the day is ruled by a planet. I was born on the day of Jupiter in the hour of Mercury.
Remember though that this system is different from our modern perceptions. It's based on the day beginning at Sunrise. So the first our of the day on a Thursday is ruled by Jupiter and the planetary hour rulers continue in Chaldean order throughout the day - i.e. the next hour is ruled by Mars, then Sun .... The sequence repeats i.e. hour 8 is back to Jupiter. If you follow this through you'll find that the first hour of Friday is ruled by Venus, the first of Saturday, Saturn, etc.
By the modern calendar I was born during the first hour of Friday (ignoring daylight saving), by the traditional Astrological calendar I was born during the seventh hour of night on a Thursday.
The 'hours' are not our equal 60 minutes but an equal division of day time (Sun from Ascendant to Descendant) and an equal division of night time (Sun from Descendant to Sun at Ascendant). So Day hours do not equal the same length of time as night hours, except at the equinoxes (equal night and day).
A traditional reading of any chart would pay some attention to the Day and Hour rulers, if these were well placed in the chart it would be a good sign. But it's not a major thing.
