Time in a nutshell

Time is something we experience on a daily basis; characterised by notions such as the past, present & the future. We try to follow time with clocks and calendars and no matter what we do it still keeps passing in an uniform single direction heading towards the future.

Science books define the unit of time - "second" by the electronic transition of the Cesium atom that itself doesn't add more to our understanding of time. But when physicists are forced to define time in Understandable way they define it as "the progression of events from the past to the present into the future.
Basically that means, if a system is unchanging, is it timeless?

Time can be considered to be the 4th dimension of reality where reality comprise of 3-dimensional space. Time in the natural world has only one direction but our physics equations work equally well whether time is moving forward (+ve time) or backward (-ve time). 

Can we consider this as a hint that somewhere in parallel universe time can move in both the directions?

We can't answer this question now but though we can give explanation for the irreversibility of time, i.e - our world follows the laws of thermodynamics & 2nd law of thermodynamics states that within a closed system, the entropy of the system remains constant or increases. If the universe is considered to be a closed system, it's entropy (degree of disorder) can never decrease.

In other words, the universe cannot return to exactly the same state in which it was at an earlier point, I.e
Time cannot move backward.

Apart from this strange characteristic about time, it encompasses such more strange characteristics:

Time dilation

In classical mechanics, time is same everywhere and synchronized clock remain in agreement. But according to Einstein - Time is relative & it depends on the frame of reference of an observer and this can result in time dilation which means that the time between events become longer(dilated) the closer one travels to the speed of light.

Moving clocks run slowly than stationary clocks, and the effect become more pronounced as the moving clock approaches light speed.

(For eg:- clocks in jets or in a orbit record time more slowly than those on Earth).

Time travel

Means moving forward or backward to different points in time same as we move across points in space. Till now we have observed jumping forward in time. Astronauts on the space station jump forward in time when they return to Earth as it's slower movement relative to the station.

However, travelling back in time has some problems. One issue is causality or cause and effect. We can understand this by a famous paradox called "The grandfather paradox". According to this if you travel back in time and kill your own grandfather before you mother or father was born, you could prevent your own birth.

Many physicists believe time travel to past is impossible but some presume there are solutions, such as travelling between parallel universes or branch points.

But how it all got started?

According to the general theory of relativity, space, or the universe emerged in the Big bang around 13.7 billion years ago. Before that all matter was packed into an extremely tiny dot. Before Big bang there was no space or time.
We can measure cosmic background radiation as microwaves from the big bang, but there isn't any radiation with earlier origins.

How we perceive time?

Our human brain is equipped to track time and is responsible for our daily sleep-wake cycle or circadian rhythms. But this perception of time by our brain can be affected by neurotransmitters and drugs.
Chemicals that excite neurons so that they fire more quickly than the normal speed up time, while decreased neuron firing slows down time perception. Basically when time seems to speed up, the brain distinguishes more events within an interval, i.e time truly does seem to fly when one is having fun.

Scientists believe that memory formation is the basis for human perception of time, the brain forms more memories of new experiences than of familiar ones. The same phenomenon explains why older people seem to perceive time as moving faster than when they were younger. Since fewer new  memories are built later in life, time seems to pass more quickly.

And anything beyond our present understanding about time can only be answered when the right time will come.