It should also be noted that humans do not apply time-discounting equally over all outcomes and are also risk averse. That's why I have a hard time to accept arguments against discount rates based on the decisions of some hypothetical agent. Humans are not going to attempt time-travel, because they are risk-averse. Further, time-travel is subject to time-discounting as well. Just because you might be able to turn the clock back doesn't change anything about your subjective time experience, which is the basis of human time preference.