Time is a mysterious concept. In Einstein's account of space-time as a unified concept - the only way time differs from other spatial dimension is via a minus sign. While physicists have a much better understanding of time now there are still many open questions when it comes to time-travel. While as per relativity traveling to the future is quite simple (all one needs to do is travel fast) it is still not clear if time-travel to the past is possible due to many paradoxes it poses. For an interesting account of time using simplified euclidean geometry take a look at this.