British novelist HG Wells had imagined a "Time Machine" in 1895, which was to unleash a flurry of films and books on this subject thereafter, but the enigma of time travelling has remained intact.
For instance, if a shot is fired from your end, you can see it coming to you only when you run faster than the speed of bullet and then look behind, and you can do it only when you can travel in time. The span of time makes you clairvoyant, it gives you a vision into future, but sadly no such Time Machine is made yet.
However, in Hindu mythology, we find that Sanat Kumar, Narada, Ashwini Kumar etc were able to travel in time. In fact, the concept of Time Machine itself is inspired by Indian Mythology. For instance the myth of Raivata. He had gone to Brahma Loka to meet Brahma but when he returned back to Earth, four ages had gone by since one day of Brahma is equal to four ages of Earth. Raivata had gone there to seek suitable groom for his daughter, but when he returned, even the grandchildren of his daughter were already dead.
There is a scientific reason behind this. In space the measurement of time is different from our Earth. One year of Earth is 365 days but one year of Mercury is only 88 days, while Neptune's one year is 164.8 years long! So, what is the measurement of time?
Scientists say that once something happened, it can't do undone ever again. This also means that if this cosmic record can be accesses somehow, time travelling will be possible. However, one can change one's future by the virtue of hindsight of time travelling but the past can never be changed.