I dont know who.. but... A "day" is not arbitary in the slightest, it's the time it takes for the earth to complete a 360 degree rotation. The time it takes to orbit the sun is a year - this is the time it takes for it to complete one entire orbit. There are 24 hours in a day because the earth has a circumfrence of about 24000 miles, and we spin at 1000 miles per hour (notice, nice number), therefore 24 hours = 1 complete spin.

A "day" is the time from noon (when the Sun is at zenith) to noon. 365 and a quarter or so is the actual number of days before the Earth returns to its original spot, and you can tell this because it takes this number of days between the Vernal (spring) equinoxes. The Vernal equinox is the time when the Sun appears to cross the Earth's equator from the South to the North. If you live on the equator, this is easy to measure, otherwise I think you need to calculate. But you would notice after a few decades that your calendar was getting out of step with the seasons if you do not put in leap years. As for the 24 hours, the first people to invent the our idea of an hour were probably the Ancient Eygptians who used a counting system based on 12 and 60, instead of 10 like today. They naturally for them divided the daylight into 12 equal pieces. The length of these ancient hours originally varied with the amount of daylight and was a rough measure. "it's the third twelfth of the day!" As soon as primitive clocks (such as water clocks) are created, it is necessary to have a standard hour which is 1/12 of the average amount of daylight, and then there are also exactly 12 of them at night as well making 24 hours.

I like the Egyptian hypothesis, Egyptian found many things who influence our lives. For the rest, i already know it, i'm not stupid. I just asked my self why we count time, who have this idea? And why or where it was decide that 60 seconds is 1 minute, 60 minutes is 1 hour, 24 hours is 1 day, and so on ...?