### Theory:

Units of time

Time is measured in the units of $$second$$ ($$s$$), $$minute$$ ($$min$$) and $$hour$$ ($$h$$). The larger units of time are $$minutes$$ ($$min$$) and $$hours$$ ($$h$$) whereas $$second$$ ($$s$$) is a smaller unit.

Important!
All the units should be written in singular form and not in the plural.
Example:
The value should be given as $$10\ min$$ and not $$10\ mins$$.
Also, the units of time vary depending on their need.
Example:
1. Age is generally expressed in years and not in days or hours.
2. The time taken to travel between two places is expressed in days or hours and not in years.
3. It takes around one $$second$$ to spell 'one thousand and one' aloud.
4. The pulse of a human heart beats about $$72$$ times in a $$minute$$. This is about $$12$$ times in $$10$$ $$seconds$$. This pulse rate is higher for children.
The time period between two sunrises was called a day.
$$1\ \text{day} = {24\ \text{hours}}$$
Likewise, the time interval between one new moon to the next new moon was called as a month.
$$1\ \text{month} = {30\ \text{days}}$$
The time taken by the Earth to complete one revolution around the Sun is known as a year.
$$1\ \text{year} = {365\ \text{days}}$$
$$1\ \text{year} = {366\ \text{days}}$$ ( For leap year)
Microsecond and nanosecond

One second is the smallest known interval of time that is available in clocks. Improvement in the technology has led to the development of clocks that can measure the time intervals smaller than a second. It can even measure units like microsecond and nanosecond that are smaller than a second.
One-millionth of a second is equal to one microsecond.
One-billionth of a second is equal to one nanosecond.

The clocks that measure microsecond and nanosecond are used for scientific research. These types of clocks are also used in sports to measure time that are one-tenth or one-hundredth of a second. The terms 'centuries' or 'millenniums' are used to state historical times. The lifetime of stars and planet are represented in billions of years. From this, it is understood that time can be measured from nanosecond up to billions of years.

National Physical Laboratory in New Delhi uses a clock of accuracy around one-millionth of a second ($$microsecond$$). National Institute of Standards and Technology (U.S.A) has created the most accurate clock in the world, which lose or gain one second after $$20$$ million years.

Speed measurement

Speed is measured in the units of $$cm/s$$ or $$m/min$$ or even $$km/h$$ since, $$\text{Speed} = \frac{\text{Distance travelled}}{\text{time}}$$
The speed of an object can be calculated by measuring the time and distance. The speed of an object can be compared while walking or cycling with someone. By measuring the time taken to cover a certain distance, we can calculate the speed.

The speed of some living organisms may not be the same for all.
Example:
Tortoises move very slowly with a speed of about $$8\ cm/s$$ whereas, on the other hand, the rockets that are used to launch satellites into Earth’s orbit travels at a speed of $$8\ km/s$$. The difference between the speed of the rocket and the tortoise is large in number.

Important!
To convert the unit from $$m/s$$ into $$km/h$$, multiply the given distance value by $$18/5$$.
To convert the unit from $$km/h$$ into $$m/s$$, multiply the given distance value by $$5/18$$.
Distance calculation

The distance moved by an object can also be calculated by knowing the speed of an object. The formula is given by,
$$\text{Distance covered} = {\text{Speed of an object}}\times{\text{Time taken}}$$

Time calculation

The time taken by an object can also be calculated with the help of speed and the distance covered by an object. The formula is given by,

$$\text{Time taken} = \frac{\text{Distance covered}}{\text{Speed of an object}}$$

Speedometer
Speedometer is a device that records the speed in terms of $$km/h$$.

Odometer is a device that measures the distance moved by them.
These two devices can be seen in the dashboard of vehicles like buses and cars. With the help of these devices, the distance and the speed of a vehicle can be easily found.

Reference:
https://images-na.ssl-images-amazon.com/images/I/61sd%2BkcWy2L._AC_UX425_.jpg