There is no true time. Suppose we calibrate our clock using a pendulum. How do we know it swings in regular intervals?

There is no true time. Suppose we calibrate our clock using a pendulum. How do we know it swings in regular intervals? If we try to confirm that by counting the oscillations using our pulse, for example, but how then do we know our pulse is regular? We might compare my pulse to another pendulum, and this just gets all circular.

You might ask, well I can count the seconds on my watch (which depends on pendulums) and it seems regular to me! But our senses could be fooled or proceed in a non-regular way, and that, is as hard to find as tricks played on us by our own brain.

We really don't know, it just seems to be useful assuming such regular & universal "t" exists.

 If we try to confirm that by counting the oscillations using our pulse, for example, but how then do we know our pulse is regular? We might compare my pulse to another pendulum, and this just gets all circular.

You might ask, well I can count the seconds on my watch (which depends on pendulums) and it seems regular to me! But our senses could be fooled or proceed in a non-regular way, and that, is as hard to find as tricks played on us by our own brain.

We really don't know, it just seems to be useful assuming such regular & universal "t" exists.