What is a length? In days gone past, people use to compare size of objects to what they immediately had at hand: themselves or parts of their bodies. The British Imperial Units have this origin.
For length, the inch is approximately the thickness of ones thumb, the hand, the thickness of ones hand, the foot the size of ones foot. The yard was defined as the distance of ones nose to ones outstretched thumb. The step is the distance of 1 step and the pace the distance of two steps (so you return to the foot you were originally standing on). The Roman Empire originally defined the mile as 1,000 paces.
Of course people come in all shapes and sizes, which gave great deviation in accurate measurements. To account for this, standardisation was required. One of the earliest standards would have been the most powerful man alive the king or ruler. A carpenter would have sized up the king and made a wooden stick with markings. This wooden measurement device is known as a "ruler".
Time, have you ever wondered why there are 1,000 milliseconds in a second, 60 seconds in a minute, 60 minutes in a hour, 12 hours on a 12 hour clock, 24 hours in a day and 365 days in a year?
The babylonians had a base 60 system where one looked at their four fingers (excluding their thumb) and seen that they were viably separated into three segments. One could use one hand to count up to 12 and two hands to count up to 24. The fingers on the second hand could however be used to denote multiples of 12 which would sum to 60.
Below a second, the modern decimal system is used.
The decimal system is based on using 10 characters to represent all numbers; 0,1,2,3,4,5,6,7,8,9 – all other numbers can be made using arrangements of these numbers*. This is called base 10.
* The decimal point is also required as part of all numbers. Some numbers cannot be made perfectly using the decimal system such as 1/3 and have digits which recur forever.
Double click the chart below to view it in more detail.
There are 7 days in a week. These correspond to the "7 planets" known at the time, including the sun and moon. The sun and the moon being the most important which is why they come first in the week (except in America). In old English/Norse Sunday was Sun's Day corresponding to the Sun, Monday was Moon's Day corresponding to the Moon, Wednesday was Woden's Day corresponding to Mercury, Thursday was Thor's Day corresponding to Jupiter, Friday was Freya's Day corresponding to Venus and Saturday was Saturn's Day corresponding to Saturn. In Latin Tuesday was Tiu's Day corresponding to Mars.
If one looks at the days of the month we can see they have a non-standard number of days and the months we call October, November and December correspond to the 9-12 months when Oct, Non and Dec correspond to the numbers 8-10.
30 Days has September, April, June and November.
All the rest have 31.
Except February Alone which has 28 Days Clear and 29 in Each Leap Year
In King Romulus' calendar Martius was originally named after God of War, Aprilus named after the Goddess of Love, Maius named after the Goddess of Spring and Lunius named after the Goddess of Marriage and Childbirth. The rest were essentially named 5-10. This calendar had only 304 days and seasons didn't quite align from year to year. It was improved upon by King Numa who added Januarius and Februaris to the top of the calendar corresponding to the God of Beginnings and Feast of Purification. Unfortunately this messed up Quintillis-December which called 5-10 but correspond to months 7-12. This calendar was 355 days and still didn't quite align with the Earth's planetary rotation. The calendar was improved by Emperor Julius Caesar and his successor Emperor Augustus. Quintilius and Sextilius were renamed July and August in their honour. This calendar is known as the Julian calendar and has 365.25 days corresponding to the Earth's planetary rotation. It is currently used.