I read this book in the 70s and was quite an old book then maybe 60s or earlier. I have read The End of Eternity but there were similarities but the male female main characters were more naive.
Time travel was like an elevator and the main characters (man and his girlfriend) travel to the distant future and distant past. I remember the extreme future mankind has gone but I remember the planet was sentient.
Part of the story revolved around a blockage in a section of time, this turned out to be blocked by an alien presence from one of our gas giants. These aliens were also trying to stop humans from developing space travel. Not much to go on but I also remember that the temperature in space was a theme used to stop the development of space travel.
The story started in the near future for the book, time travel was available but space travel had been discouraged and I think this was on purpose and was related to the future blockage in time. I do remember the main characters traveled to the distant past (encountering dinosaurs) and the distant future were mankind had died out. I think they were running from something.