When did humans start using time?
As soon as there was time, people were using it, dividing their activities according to day and night, for instance.
What is the explanation for the difference in time?
The earth rotates on its polar axis once in every day - or 24 hours. So over a 24 hour period it turns through 360 degrees. This every time you go 15 degrees of longitude (= 360/24) the solar time is 1 hour earlier or later. The solar time has been modified so that most countries, which do not stretch very far from East to West use the same time throughout.
How many times does the hand go around the clock before you know the minute is up?
The second hand goes around once.
How many minutes will it take to count to a billion seconds?
Assuming you could average a number per second (not easy if you enumerate some of the larger numbers fully) one billion seconds is 16,666,666 minutes and 40 seconds. It's a little over 31 and 1/2 years, if you never sleep.
666.66 hours is 666.66 hours long. It's 27.775 days, which is a little less than a month.
What is considered afternoon and evening?
Afternoon is the time period from midday (12:00 noon) or the midday meal to around 6:00 pm or sunset. Evening is the time period which follows, and it lasts until "normal" bedtime.