Put simply, time is that which a clock measures in much the same way that spatial distance is that which a ruler measures. The key differences being that we cannot move freely back and forth through time like we can through our spatial dimensions and we cannot be stationary along the time axis. It is currently unknown why this is so.
Unfortunately, past experience has shown that threads on time itself tend to be of poor quality, full of philosophical musings that are unanswerable, and attract personal opinions and crackpots like honey attracts ants, so I'm going to have to lock this thread.