Time is an 'illusion' - it is a fabrication by humans. It is controlled by precision, scientific equipment the world over. It is not something that is a natural, physical property such as gravity. We did not define what gravity was (although we did name it 'gravity'), but we did define, over millennia, what time is. We defined the definitions of past, present and future.
Before the invention of clocks and calendars, we just existed. We noted days and nights, with the rise and fall of the sun and moon, but this was just the way things were - a cycle of life. It wasn't until civilisations, such as the Mayans, started breaking these cycles down into their calendars and recorded them, and people like the Ancient Egyptians started using sundials as a primitive timepiece, that 'time' as we know it, with seconds, minutes, hours, days, weeks, months and years started to come about, and then we defined the past - events that happened prior to the here and now; the present - events that are happening in the here and now; the future - events we perceive will happen beyond the here and now.
What we also have to bear in mind is that timekeeping, especially during the modern era, did not simply emerge; it was imposed. Even the current WTZ (World Time Zone) system as we know it today, only came about at the beginning of the 20th Century and during a period referred to as the 'Battle of the Clocks' which lasted almost half a century prior.
'Time' can also be 'selective' depending on an individual's view. In 46 BCE, Julius Caesar reshuffled the Roman calendar to insulate it from the priesthood ('one especially consecrated to the service of a divinity and through whom worship, prayer, sacrifice, or other service is offered to the object of worship - and pardon, blessing or deliverance is obtained by the worshipper' [Source: Funk and Wagnall Vol. 21 p.273]). Joseph Stalin thought that the weekend was a middle-class luxury; he abolished it in 1929 in an attempt to transform ordinary Russians into good Communists.