Time is not a reality (hypostasis), but a concept (noêma) or a measure (metron).---Antiphon the Sophist (5th Century BC)
. . . fast forward to our century and this is about the best definition one will ever see . . .
Time: the equation of motion to a preconceived cycle of motion that is assumed to occur regularly.
Time is a relation. First we conceive of regular, repetitious motion. Then we relate other irregular action to the regular cycle and by counting the repetitions of the cycle, we derive a number that can be considered "time".
Seconds, days, years, etc. are all abstractions referring to regular, repetitious cycles that we have conceived of as constant and dependable.
Even atomic clocks, which are considered "the most precise" are based on the assumption that the atoms of the clock are performing an action in cycles that are regular and dependable.
Time is based on assumptions and artificial, idealized relationships. ---Mike Huttner (21st Century AD)
For Time to qualify as real it would have to have form and stand out implying three dimensions. So in other words it would have to be some invisible fundamental subatomic object, and the particle physicists and disciples of Einstein would have to spend billions to confirm its existence to the world.
But most reasonable thinkers throughout history clearly understand that Time refers to concept, a brainwork. One simply compares two or more motions. One conceives and assumes one motion as regular, cyclical, constant. And then one equates this motion to others. If there is an irregularity in relation to the assumed motion used as a standard then there should be some explanation. For example the irregularity of Earth's orbit around the Sun. Or an irregularity when comparing a caesium atom's action near Earth's surface in relation to a caesium atoms action in orbit to Earth. The presence of some fundamental invisible subatomic object in different ratios (number of subatomic objects and their effective actions) must account for the irregularity, but this object is not called Time.