define time?

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future.
We can't know what the first unit of time used by mankind is, because the first unit of time was invented in prehistoric times, which by definition makes it pre-historic (i.e., before any historical records could document it). After consulting a copy of Calendars in Antiquity: Empires, States, and Societies at Google Books, the earliest recorded unit of time I could find is thehamuštum, which appears in Assyrian documents that date circa 2500 to 2000 BC. The hamuštum is approximately seven days long, but scholars disagree as to whether it was exactly seven days (like the modern "week") or an interval equal to about one-fourth of a lunar calendar month (which would make it vary between seven and eight days).