Changes

The start of a new era could be declared for any number of reasons.  Among them were the occurrence of the first or 58th year of the 60-year cycle (see below),  a new emperor (usually sometime during the year after the death of the previous emperor), natural disasters, etc.  Many periods were only three or four years long.  For most of history, the names had nothing to do with that of the emperor; though there was a Tenshô period, there was no "Emperor Tenshô."  Furthermore, a change of emperor did not necessarily mean a new period.  However, since the start of the Meiji period (1868), the only reason for a new period has been the reign of a new emperor,<ref>This was made official in 1979.</ref> and the emperor is after death referred to by the name of the period he reigned in, so now we refer to the emperors Meiji, Taishô, and Shôwa.  However, unlike the ancient reign-year system, the new era starts on the day of the death of the previous emperor (Taishô period, Shôwa period), or on the day after (Heisei period), not from the next year.

The start of a new era could be declared for any number of reasons.  Among them were the occurrence of the first or 58th year of the 60-year cycle (see below),  a new emperor (usually sometime during the year after the death of the previous emperor), natural disasters, etc.  Many periods were only three or four years long.  For most of history, the names had nothing to do with that of the emperor; though there was a Tenshô period, there was no "Emperor Tenshô."  Furthermore, a change of emperor did not necessarily mean a new period.  However, since the start of the Meiji period (1868), the only reason for a new period has been the reign of a new emperor,<ref>This was made official in 1979.</ref> and the emperor is after death referred to by the name of the period he reigned in, so now we refer to the emperors Meiji, Taishô, and Shôwa.  However, unlike the ancient reign-year system, the new era starts on the day of the death of the previous emperor (Taishô period, Shôwa period), or on the day after (Heisei period), not from the next year.

As mentioned above, a change in the era name can occur at any time of year.  For instance, the Tenshô Era began the 28th day of the Seventh Month of 1573 and lasted until the 8th day of the Twelfth Month of 1592.  New periods began during the day. However, the present period, Heisei 平成, began at exactly at midnight the morning of January 8, 1989.  The exact day an era begins was not considered important until recently, however. At times, as in the Edo period, eras were treated as having started at the beginning of the year retroactively.  Furthermore, as one never knows when a new period will begin, one often finds dates that officially did not exist. For example, there is a grave in western Tokyo (formerly  [[Musashi province]] ) dated  "Shitoku 4 (1387), 11th month," though Shitoku had ended in the 8th month. (Note that this is a "northern" ''nengô''.) In modern times, drivers licenses valid to "Shôwa 66" remained valid to 1992, though Shôwa ended in 1989.  

As mentioned above, a change in the era name can occur at any time of year.  For instance, the Tenshô Era began the 28th day of the Seventh Month of 1573 and lasted until the 8th day of the Twelfth Month of 1592.  New periods began during the day. However, the present period, Heisei 平成, began at exactly at midnight the morning of January 8, 1989.  The exact day an era begins was not considered important until recently, however. At times, as in the Edo period, eras were treated as having started at the beginning of the year retroactively.  Furthermore, as one never knows when a new period will begin, one often finds dates that officially did not exist. For example, there is a grave in western Tokyo (formerly  [[Musashi province]] ) dated  "Shitoku 4 (1387), 11th month," though Shitoku had ended in the 8th month. (Note that this is a "northern" ''nengô''.) In modern times, drivers licenses valid to "Shôwa 66" remained valid to 1991, though Shôwa ended in 1989.

===Calculating Year Periods===

===Calculating Year Periods===