The simplest truth about time is that it is not what we commonly think it is.
We think of it as being a constant flow; like a drum-beat against which the song of the universe plays. But really it is a dancer moving to create the rhythms required by the universes’ song.
Ontology of Time
What is time? It is a byproduct of motion.
When one object moves relative to another, we can perceive time. If no objects move in any way, there is no perception of time. The universe itself has safeguards built in to protect against time stopping: the Sun constantly moves, at a fairly steady rate, relative to the Earth – and the non-stop motions of the Moon and 5 or 6 other heavenly objects add poly-rhythmic richness to the time flow built into fabric of the universe. Thus the Vedas, Puranas, Siddhantas et.al (and equivalent works from every human, including our modern civilization) measure time relative to the movements of the Sun, Moon, etc. The movements of the Sun and planets actually create our perception of time.
What is movement? It is change.
Why is there change? Because there is karma (destiny).
Why is there karma? Because there is independent will.
Why is there independent will? Because there is ignorance.
Why is there ignorance? Because there is freewill.
Why is there freewill? It is free, self-creating, lot dependent on another cause.
Time is a byproduct of freewill. Time as we experience it in the universe is a byproduct of freewill in an ignorant condition, operating quasi-independently from the fountainhead of freewill: Godhead. Such independence means independent responsibility, the agent of which is karma (destiny). Changes occur (and thus time is perceived) as the universe moves to fulfill the conditions required by destiny.
Time is therefore pushed forward by the collective desires of living entities, forcing the universe to move into shapes and conditions according to their desires.
Elasticity of Time
Time is not entirely rigid or constant.
Einstein believed that the speed of light creates a constant time-clock for the universe. This is really a brilliant conception because light is a form of the Sun, and the speed of light is a measure of the movement of that form of the sun relative to an observer. Einstein noted & demonstrated that the observer’s motion relative to light changes the perception of time itself.
Movements of the Sun (including the speed of light) create rigidity and consistency in universal time. But these are not absolute or insurmountable.
This is why different beings who have different relationships to the movement of the Sun / light have different conceptions of time. Time thus flows differently in different “lokas” (lokations).
Various “Time-Zones” in the Universe
The Sun creates a day by moving around the earth once… because it spends half the time above it creating a bright period and the other half below it creating a dark period. So we observe a change between day and night. The change created by this motion allows us to perceive a certain duration of existence as a “day.”
This is true for us humans, from our point of view. But other beings live at a locus (called a loka in Sanskrit) that doesn’t relate to this movement. In the netherworld called “Yamaloka,” for example, they relate to the Moon’s movement relative to the Earth and Sun. When the Moon is on one side of the Sun (waxing) it creates a bright period, and on the other side it creates a dark period (waning). These beings experience a ‘day’ to have as much raw time as our locus grants us within an entire month!
Still other beings have yet another relationship to the Sun and light. In paradise, the “Sura” (gods) relate most directly to the Sun’s movement through the stars (which we translate to the Sun’s movement relative to earth’s equator). When the Sun is among stars equivalent to being northward of our equator, a bright period occurs (uttarayana). When it is among stars equivalent to it being southward of our equator, a dark period occurs (dakshinayana). These beings have in just one day all the span of time that is available to us in an entire year!
There are still longer periods for higher demigods, culminating in the extremely vast timescale of Brahmaloka – the highest locus available within a universe – at which a single “day” has within it all the moments that are available to our human mind only in the course of more than 8.6 trillion years!
This timescale is not relative to the movement of the Sun at all, so much as to its existence and destruction!
The ultimate generator of timeflow is the prana (“breath”) moving in and out of Mahavishnu. The inhalation is like “night” and the exhalation is like “daytime.” This period of time is transcendental to the Sun and light. It continues to allow change even when Brahma’s locus disintegrates and the universe dissolves.
Going Back in Time
Time is flexible but not completely. In all cases, it flows forward. It flows at flexible rates, but always in the same direction. This is the verdict of human culture, the Vedic records, and Einstein also demonstrated it in his equations pertaining to the relativity of time. Time can slow to almost a standstill, but it never flows backwards.
Still, “forwards” and “backwards” becomes a little bit irrelevant when you are talking about cycles. For example, winter comes after spring but also comes before it. Or another example: a chicken comes before an egg, but an egg comes before a chicken. These conundrums are simple truths because time flows forward in a circular motion.
It is not possible to move backwards in time, but if you move far enough forwards you eventually come to very, very similar conditions as existed in the past.
It is for this reason that some mystics say “past” and “future” are misleading concepts.
Multiple Realms in Multiple Universes
The Puranas describe Mahavishnu as lying in an ocean of causality, exhaling egg-like bubbles which each develop into an independent universe. Each universe has its own timeline, that mirrors the others but is not identical to it.
Within each universe there are many “dimensions” or “parallel realities” that are somewhat related and have intersections and access points at various locations under various conditions. Thus there are parallel realities transpiring within each parallel universe.
“It is always noon somewhere on earth.” Similarly there is always a certain point in history extant somewhere among the universes. But we cannot hop skip and jump between universes, we are confined by what we perceive as the vast, vast distances between them.
Each universe is a closed system in which there are souls (jiva) with independent will creating destiny and motion. A particle cannot leave a system while it is still in motion within that system. Souls in a universe remain there until it is dissolved or until they dissolve their independent will, end their destiny, and become eligible for liberation (moksha).