How can time be simultaneous?

Simultaneous means "at the same time" so, It is by definition.

Yes, it is possible , the past , the present and the future of the sun, that is it existed, is simultaneous ,since it is the same sun in itself , nothing change. When it is relative to something then, the time, the place and the condition would change, so the past, the present and the future would come in to play. 

This might help ...

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In a four dimensional reality like ours (length, breadth, height and time), it does not make sense to say all time is simultaneous. In 3D + time, we can create a plane and, by looking along its edge, see it as a line. Suppose we had a line and could look at from the end. It would look like a point. Imagine an additional dimension that enabled us to look at time from an 'end-on' perspective. It would form a single point in which all time existed. As humans, we only know reality as 3D + time though I understand that many of the mysteries of the universe can be explained if there really are more dimensions that exist but that we cannot see or measure. Mathematicians are able to model such complexity but, as a non-mathematician, I can't picture it. For all practical purposes, there are three dimensions plus time and, within that model, we experience the passage of time. What has gone has gone. The future does not exist yet. The only place the past, present and future exist simultaneously is in out heads as concepts. No one has shown the existence of 'multiverses' with multiple time streams running simultaneously.

It’s called Block Theory. a theory which states that the past, present and future exist simultaneously.

In other words, this means that once an event has occurred, it continues to exist somewhere in space-time.

The new ‘controversial’ theory is backed up by Albert Einstein’s theory of relativity which indicates that space and time are in fact part of an intricate four-dimensional structure where everything that has occurred has its own coordinates in space-time.

Time is related to velocity, velocity is related to distance. Past, present, future are the distance of time velocity. Try to imagine time without velocity. What is it?