Who rules the time? Reflections upon the role of conventionalism in time measurement

We try to rethink and actualise conventional understanding of time measurem ent. We discuss historic development of the "ideal" clock - one that measur es time most accurately - through the history of natural sciences. We defen d thesis that scientific revolution of the seventeenth century marks the es sential rupture in the understanding of time measurement. In their late wor ks Johannes Kepler and Galileo Galilei developed a new concept of time, the measure of which does not depend on specially selected particular natural movement (usually it was the rotation of the sky). At that historic moment time became a physical quantity, which is rationally defined by universal m athematical laws of Galilean nature. Time reflects parameter t from equatio ns of mathematical physics as accurately as possible. This change in the co ncept of time was probably one of the key turns that made modern mathematic al physics possible.