Trying to recall the title and author of a time travel story that uses the terms "uptime/downtime" to indicate past/future (not sure which was which). The protagonist was able to travel in either direction with equal ease. One attribute of future time travel was that the environment got visually thinner, i.e., lacking in solidity, the further into the future the protagonist went. This story was written no later than 1970. Thanks for your help.