Time dilation, the bane of temporal tourists. You see, when you approach the speed of light, time gets all wibbly-wobbly.
Don't even get us started on this. You think you can just hop into a DeLorean and zip around the timestream, but the math gets all Lorentz-y on you. Gamma factor, beta factor, all that jazz.
Gamma = 1 / sqrt(1 - (v^2 / c^2))
Where v is your velocity, c is the speed of light, and gamma is your gamma factor. Don't ask us what it means, just trust us, it's complicated.