First Ocean, 12th Day of Childwinter, 526 M.E. (Helios): Time is existence, according to Zen Master Dōgen, and all existence is time. When we are waiting for someone, an hour can seem very long, but when we are pressed for time, an hour can fly right by. The passage of time is relative but we still measure it as exactly 60 minutes per hour and sixty seconds per minute. 

It's commonly assumed that time always moves forward at the exact same rate, one second per second, and this happens everywhere, always, in all instances. You can set your watch by it. If one hour seems excruciatingly long and another seems to zip right past, we assume it's our perception, not time, that's been altered. But if time is existence as Dōgen asserts, and existence is subjective and relative, then time is actually our subjective experience of it, and some hours, minutes, and seconds are truly longer than others. One hour doesn't merely seem longer than another; one hour is actually longer than another.

Einstein proved time is relative to the observer by mathematics that I don't understand, but I can summarize my interpretation using the old joke about the chicken crossing the street. 

I don't know why the chicken crossed the street, but there was a time when the chicken existed on one side of the street, another when it existed on the other side of the street, and a time when it existed somewhere  in the middle of the street. We can think of time as a linear process, a series of instances that arrives at the present moment and then moves on. That sequence of instances exists now, and our existence occurs in that "now."

We can see time as that linear progression, but since existence is time and vice versa, the chicken's existence and time were always one and the same. Time never "arrived" at the chicken's existence, which is to say, the chicken never existed outside of time. Assuming it didn't get hit by a car or something and it still exists, time never left the chicken. Time does not come or go from existence, and when we see time as a constant, as opposed to a linear progression, then even a continuous process, like crossing the street, is seen as the present moment.

Let me try putting it another way: the chicken on the other side of the street was in the past and no longer exists except as a memory. The crossing of the street only exists in my mind, which put together a string of memories and observations, and since my mind is always in the now, the chicken's crossing is an instantaneous moment, occurring right now.