T-Rex: The time: the past! Or so you assume...

T-Rex: ...because it's secretly THE FUTURE!

T-Rex: But it's a future where everything went bad and knocked us all back to history times, and then everyone forgot how things used to be better when we had planes and air conditioning and stuff. It's just completely forgotten, and the evidence was destroyed, and all the records of it were lost too! Dang!

Utahraptor: So it might as well literally BE the past since nobody IN the story, or READING it, can tell otherwise.
T-Rex: Aha!

T-Rex: So you assume, BUT: when the characters dig for unrelated reasons and NEVER encounter oil within the shallow depths one can reach without industrialization, the SAVVY reader realizes that it could only be the future, since those resources were all exhausted by the present day! And they go "oh neat."

Off panel: And the rest of us, in an otherwise pointless digging-a-hole-for-funsies scene, never notice anything is wrong.
T-Rex: It's writing 101!! The best worldbuilding is subtle, AND relies upon depleted oil reserves