Philip José Farmer’s 1972 Time’s Last Gift is a time-travel novel. Revised in 1977, Time’s Last Gift shares continuity with Farmer’s Opar novels.
With a thunderclap, a time machine (H. G. Wells 1) appears in Stone-Age Europe. Geography having changed more than predicted between the 2070 from which the machine departed and the 12,000 BC when it arrived, the time machine immediately tumbles down a hillside. Thanks to prudent design, all of H. G. Wells’ occupants — beautiful Rachel Silverstein, disappointing Drummond Silverstein, German Robert von Billmann, and mysterious he-man adventurer John “not Tarzan” Gribardsun — are unharmed.
The explorers have four years in which to document Europe’s Stone-Age culture. Due to the way that time travel works (in this novel at least), nobody will ever be able to visit this era again. The scientists plan to make the best of those four years. Gribardsun has his own plans, which he does not care to share at this juncture.