universes both with and without concepts of time, the natures of which continued to be the subject of long discussions between himself and Einstein. Although in 1949 Gödel postulated a theorem that stated: "In any universe described by the theory of relativity, time cannot exist", it is important to stress that Gödel was considering many different universes employing for their theoretical constitutions many different theoretical conceptions of time. It was his endeavour to reconcile the paradoxa of time and timelessness with relativity theory upon a sound epistemological footing. Hence when Gödel is popularly known for proposing "his" universe in which time-travel is a possibility, it is in reality a gross overreduction and oversimplification of his work. Calendars in timeless universes In a universe where time does not exist, the measure of time is spoken of only as existing within the sphere of man-made society and its sciences. The regular motions of the sun, earth and moon are real, and the shadows they produce can hence be measured and artificially quantified into days, months and years, and subdivided using other movements for measurement into increasingly small subdivisions. This is done for convenience in human societies for purposes of measurement or comparative analysis in history or other sciences. It is therefore still possible in a timeless universe to speak of a date in time, such as the Battle of Marathon in 490 B.C., as long as such that time is clearly separated from physics, and confined to the sphere of man-made historical science and its conventions. That is, when we understand that the year is a measure for revolutions of a planet around the sun, and that our planet was measured by historians as having completed some 2510 revolutions since that event occurred, which we may read out from our catalogues that fix events to observations of physical revolutions. No real time is measured, rather observations of changes in seasons and nights and days are recorded by historians. References Ontology Time