Where does that sit in geological time?

Deep time is the hardest scale to hold in your head, because nothing in a human life rehearses it.

Years ago Unit

Almost exactly an end of the dinosaurs

66,000,000 yr is earlier than 4 of the 11 references on this page.

Reference figures are measurements and are rounded; the unit conversion behind them is exact.

Every reference on this page, with its size and its ratio to your number
Reference	Size	Yours ÷ theirs	Note
first cities	9,000 yr	7,333×
end of the last ice age	11,700 yr	5,641×
first Homo sapiens	300,000 yr	220×
first stone tools	3,300,000 yr	20×
end of the dinosaurs	66,000,000 yr	1×
first dinosaurs	233,000,000 yr	0.283×
Great Dying	252,000,000 yr	0.262×	The largest extinction, 96% of marine species
first land plants	470,000,000 yr	0.14×
Cambrian explosion	538,000,000 yr	0.123×
first life	3,700,000,000 yr	1/56.1
formation of the Earth	4,540,000,000 yr	1/68.8

Stegosaurus is further in time from Tyrannosaurus than Tyrannosaurus is from us — 83 million years against 66.
Life appeared within about 800 million years of the Earth forming, but stayed single-celled for the next three billion.

Frequently asked

66 million years ago, at the Cretaceous–Palaeogene boundary, following the Chicxulub impact. Birds are the branch that survived.

Radiometric dating of minerals in volcanic ash layers, mostly uranium–lead in zircon, which is accurate to well under 1% at these ages.