Back in time impossible, forward in time maybe.
According to Albert Einstein
Time dilation is permitted by Albert Einstein's special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well.For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole.
seems like you can travel forward and backward too
You can't. It's only 'theoretically possible', but not literally and never will be. Hawking says you can't as well:
http://en.wikipedia.org/wiki/Chronology_protection_conjecture 
Hawking, Hawking - what he knows...
1st proof you can time travel - Satoshi Nakamoto did travel from future to our times to give as the Bitcoin.
2nd proof -
when the night is on have a look at the sky.
Look at the stars, how nice they are - but this is a past.
What you are staring at can possibly not exist anymore.
This is only the light traveling in space... this object - this "star", "planet" or anything else what you see there might be no more an existing object.
Now from another hand - take a swiss watch to a jet plane and travel with this watch while same watch with a same time take for a trip in a car. After same "period" of time - both watches will show different time
(~few seconds differences)
This is not "impossible" to time travel. It is just relatively hard to do
