We are a young and arrogant species. Throughout our short span of antiquity we have observed our world and tried to comprehend its meaning. Some of us observe and say, "This is so!" these are the philosophers, the governors, and the clergy. Others observe and ask "Why?" these are the scientists, the engineers, the architects, and explorers. For the "why" askers the ability to quantify and calculate the measurable elements that govern the behaviors in our existence is paramount. In their effort to try to understand and predict events in our world the "why" askers have taught us the concept of measurement. To measure something we have derived a simple comparative process. We devise something, a scale, a device, an equation; we then call this a unit, and compare what we experience against that unit to determine how many units we now have. This theory works well as long as the unit you describe is unchanged by time or the elements, universally understood, and clearly definable for everyone everywhere.
In our own simple "third rock from the sun" mentality we have derived all of our units of measure from earthly constants. A pound, a gram, a degree, a foot, a meter, or a second; these we have come to understand and have attempted to standardize and make "universal". Over time we have changed the standards, English to Metric, Fahrenheit to Celsius, and yet these standards remain based on a system that only the inhabitants of this planet, the solar system where our planet resides, or the galaxy that our solar system inhabits would understand. Where our planet exists in the universe time is a constant. This may or may not be true for other parts of our own galaxy, but time seems immutable where we live.
In our world time simply "is". Anything that has a beginning and an end has time inherently embedded in it. Every unit that we have defined as a species to measure some aspect of our lives has time associated with it. We have come to understand that time is the delta between the beginning and the end from one moment to the next. A second, a minute, an hour earthly, man-made units for sure, but they can be measured with accuracy and each one of us has the ability to say when the unit begins and when it ends. We can then answer the question of "How long?" and reproduce that length of time again.
Our lives are filled with annuals, bi-annuals, seasons, and change. Each person has a beginning and an end which happens to be approximately 70 to 80 revolutions around the sun, that's it! We call this a lifetime. I suggest that time, whatever it is, exactly, is still undefined to us in the universal scheme of things, but it exists. Like rays of infrared light, we cannot readily see them, but they exist and we have come to understand their source and meaning. Perhaps time is the simplest form of energy in the universe. As energy it exists in that moment and then is exhausted used up and expelled as events, purpose and meaning. Without time we could not get from one moment to the next. Without time would everything cease to function? Perhaps the Big Bang was something winding the universal energy clock, and swinging the universal time pendulum. What happens when we run out of time?