The standards of time-measurement which nature has given us are the year, the month and the day. Primitive observation discovered at an early date that a year was something between 365 and 366 days, and a month something between 29 and 30 days. The relation of year to day did not present much difficulty. Even the pre-Julian year of 365 days without leap-years would not produce much confusion in a single life-time. The introduction of an extra day once in four years set matters right for some centuries and the Gregorian reform by which three leap-years are omitted in four centuries will with very slight adaptation carry us on as long as the world is inhabited. So too by a system of alternations between 29 and 30 days, the month may be made to consist of a complete number of days and yet keep pace with the moon. But the relation of month to year was a more difficult proposition. Since 12 true months have approximately 354 days and 13 have 383, we have had to choose between reckoning by lunar months which run on independently of the year, and artificial months, a fixed number of which will complete the year.
