An early reference

As told by Hesiod (8th century BCE):

In the beginning, the formless confusion of Chaos brooded over the unbroken darkness. Then out of the void appeared Erebus, the unknowable place where death dwells, and Night (Nyx). Then Love (Eros) was born bringing the start of order. From Love came Light and Day. Once there was Light and Day, Gaea, the earth appeared. Gaea alone gave birth to Uranus, the god of the heavens. Uranus became Gaea's mate. Together they produced the three Cyclopes, the three Hecatoncheires, and twelve Titans.

Uranus wasn't a very good father or husband (or brother!). He hated the Hecatoncheires. He imprisoned them by pushing them into the hidden places of the earth, Gaea's womb. This angered Gaea and she plotted against Uranus. She made a flint sickle and tried to get her children to attack Uranus. All were too afraid except, the youngest Titan, Cronus.

Gaea and Cronus set up an ambush of Uranus as he lay with Gaea at night. Cronus grabbed his father and castrated him, with the stone sickle, throwing the severed genitals into the ocean. Cronus became the next ruler and ruled for many ages.

Mother Earth (Gaea) and Uranus ("heaven") begat Cronus ("time"). When Gaea made Cronus castrate Uranus, this separated heaven from earth. Then Cronus became the ruler, and thus time began.