Time Travel - Are there Rules?

[GotNoJamsss]

What is the temperature of the room when it comes to time-travel? I've always been fascinated by this trope because it's been done in 100 different ways with 9034 different rules applied. What's everyone's basic time-travel rulebook? Do you believe that going to the past has the possibility to alter the future? Or are you simply helping along the inevitable? 

What about traveling to the future? Are there consequences of any kind there?

For me, it's sort of a question of fate/choice. If you can go to the past and alter the present, then what you do in the present alters the future. However, if what you do in the past always points to the same present, then what you do now is and always has been your destiny. 

I don't know...what are your thoughts? I know there's a time-travel challenge going around so I'm really interested in everyone's thoughts!