The long unmeasured pulse of time moves everything. There is nothing hidden that it cannot bring to life, nothing once known that may not become unknown. Sophocles, Ajax
The first clock probably recognized by humans was the sky in the simplest form first by the day and night period. Various observations of the sky were used as a clock. Hesiod around 750 BC tells farmers when to carry certain activities based on the position of different stars. Time was measured for example by the apparent motion of the sun using sundials.
He also was the first discoverer of the gnomon; and he placed some in Lacedaemon on the sun-dials there, as Favorinus says in his Universal History, and they showed the solstices and the equinoxes; he also made clocks. He was the first person, too, who drew a map of the earth and sea, and he also made a globe; Diogenes Laertius, Life of Anaximander
The time flow is a mystery and physicist believe that time is an illusion. But were comes the word that “time flows”? Probably from the water clock, a tank holding water, with a very small hole in its bottom, from which the water slowly drips. The level of water sinks and its height is a measure of the time passed since it was full of water. As Heron of Alexandria says the amount of water flown is proportional to the time (as the water to water so the time to time):
κατασκευάζεται γὰρ ἀγγεῖόν τι ἔχον ὀπὴν ὡς ἂν κλεψύδρα, δι᾽ ἧς ὁμαλῶς, ὡς ἔθος, δύναται τὸ ὕδωρ ἐκρεῖν. ὅπερ προκατασκευάζεται τὴν ἀρχὴν τῆς ἐκρύσεως ἔχον ὅτε πρῶτον ἐκ τοῦ ὁρίζοντος ὁ ἥλιος τὴν πρώτην ἀκτῖνα προσβάλλει, καὶ τὸ ῥεῦσαν ὕδωρ ἐν ὧι χρόνωι ὁ δίσκος ὑπὲρ τὸν ὁρίζοντα γίνεται φυλάττεται χωρίς, εἶτα τὸ ἐφεξῆς ἐν ὅλωι τῶι νυχθημέρωι μέχρι τῆς ἑτέρας ἀνατολῆς ὁμαλῶς καὶ ἀνεκλείπτως καὶ ἀπαύστως ῥυὲν ἐν ἑτέρωι ἀγγείωι· καὶ τὸ ῥεῦσαν παραμετρεῖται, ποσαπλάσιόν ἐστι τοῦ κατὰ τὴν ἀνατολὴν ληφθέντος ὕδατος· καὶ τοῦτὸ ἔσται ἀνάλογον τῶι χρόνωι· καὶ ὡς τὸ ὕδωρ πρὸς τὸ ὕδωρ, οὕτως ὁ χρόνος πρὸς τὸν χρόνον. Ἥρων ὁ Ἀλεξανδρεύς, Περὶ ὑδρίων ὡροσκοπείων from Textus: Heronis opera quae supersunt omnia ed. W. Schmidt/L. Nix/H. Schöne/J. L. Heiberg Leipzig 1899 sqq.