+870 
This is the formula to calculate time dilatation: we consider two persons, one isn't moving at all and the other move with a velocity of v, in meters a second(m/s).
t stands for the time after the dilatation,in seconds (s), T stands for the time without the effect of the dilatation, still in seconds (s). c is the speed of light in vacuum, exactly 299 792 458 m/s.
Now, how high is t when T=10 years=315360000 s, and v=80% of the speed of the light in vacuum ?
