You can learn to astral time travel while astral projecting. You can visit both past and future realities, but because of factors yet unexplained, you cannot astral project to a specific time and place.
You can only travel to overlap points between the past, present and future realities.
Astral travelling into the past and/or future will require a little more effort than your regular astral projections.
First, you need to find a meaningful purpose and approximate time you want to visit. Once you have figured out what you want to experience, you need to have a very strong desire to visit that time period.
Proceed with the relaxation method that works for you and leave your body. Once you are out of your body, simply think of the place and time and your astral body will immediately be transported. If you don't feel like you belong, you can look at your clothing and wish it were from the right era and it will happen instantly.
If you look at old photo albums, you can definitely tell that old pictures fade with time. A similar phenomenon occurs when you astral time travel in the past. Your perception of what you see and hear will not be as clear as your regular astral travel experiences, as if the events and astral bodies had faded with time.
You can use time travel to visit and talk to loved ones that have passed away. You can also communicate with people of a different culture and language as languages are not a communication barrier on the astral plane.
The opposite happens when you astral time travel into the future. Your perception of colors and noises will be very intense, which will likely result in a very short astral travel experience.
Rest assured that you cannot alter our current reality by travelling in the past or to the future because the astral plane is a parallel plane of existence. You should be aware that astral present and physical present (as we know and experience it daily) are both real places but on different planes of existence and may be happening at different times.
Why is such time travel possible? Because of the universal consciousness, which is a realm of knowledge and wisdom. Once you get in touch with this universal consciousness, you have access to an indefinite quantity of knowledge. Perhaps this is how the prophets of our past knew about current events...
If you need help with astral time travel, make sure to review these preparations steps.