We've got it all wrong. We don't travel through time from the past into the future.
Time comes from the future.** It flows through the present and pours into the past.
Like pouring the wine out of a glass. The future is in the glass, and the past is spilled on the floor, or you drank it. You can't know for how long you can keep pouring the wine until it runs out. Clearly, you only have a certain quantity and you might accidentally spill it all out in any moment. Also, once you poured it out, you can't get it back into the glass or pour it out again. So you need to enjoy every little drop, when it comes...
*SSDD used to say that a lot back in highschool.
**I heard this idea at a grammar lecture, yesterday.