Time is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of religion, philosophy, and science, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.

In physics as well as in other sciences, time is considered one of the few fundamental quantities.[1] Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition.[2] An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.

Among prominent philosophers, there are two distinct viewpoints on time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.[3][4] The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz[5] and Immanuel Kant,[6][7] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.

Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.

There are two layman view on time.

1) consciousness is fixed, time flows

We are conscious and fixed, time is like a flowing river, bringing future events towards us, from future to ....now....and becomes...past.
or

2) time is fixed, consciousness flows

Time is fixed, our consciousness moves from past.....now...towards...future. Or so called we gain knowledge as time go by.

This is what we understand about time.

Now, let's look at what "The Guy" says:

"The distinction between past, present and future is only an illusion, however persistent" - Albert Einstein

He said both 1) and 2) about time are wrong! in his relativity theory.

For 1), time flows means time is moving. How can time move? Like a ball rolling? An object that moves must have a speed, what is the "speed" of time moving? A second of a second? Second is merely something human created, it is just a "unit" representing a segment of time. We can have 4.5 seconds to represent a "new" single unit of time called "fi". So time is moving a fi of a fi now?

Same apply to 2). What is the speed of consciousness moving, a second of a second, a fi of a fi? How can consciousness flowing? Consciousness is simply just there...

Time cannot be flowing. In other word, time does not move.

So you say, I observed an apple on a table for 3 days. Nobody moves it, so as far as space (xyz) is concerned, it is unchanged. However, I saw it get rotten more and more especially on the third day. Aren't this tell us that time is there?

This is correct, the object "apple" changed "in" time. Time is "simply there" like xyz. It doesn't move.

There is no need to distinguish time between past...now...future.

If you do not believe, try this. Say "Now". By the time you say "Now", you assume that the "Now" you just said is gone and you have to repeat saying "Now". Again, it is gone, and you have to keep repeating saying "Now" again.

You assume that every "Now" you mentioned has "flown" through time and become the "Past". And you assume that every moment is "Now". If every moment is now, what do you means by tomorrow? You have to agree that tomorrow will never come.

How Einstein explained all this is "all" events happen "in" time, just like xyz and there is a sequence of events happen and we use our calender system to record each event at different time.

As time is the medium, just like XYZ  is the space medium, it is elastic. So difference observers with different relative speed will observe the so called "now" event differently. And this is what make time travel possible in theory.

isnt that like asking what is length?

Good read, will think of something constructive to post soon. In a sense, time is well..time. Much like all other undefined things that exist today.
Apologies if I'm repeating obvious things.