Explain what the open dating system is
The official Japanese system numbers years from the accession of the current emperor, regarding the calendar year during which the accession occurred as the first year.A similar system existed in China before 1912, being based on the accession year of the emperor (1911 was thus the third year of the Xuantong period).If an end user interaction with dates and times is required, the software will nearly always convert this internal number into a date and time representation that is comprehensible to humans.The following table lists epoch dates used by popular software and other computer-related systems.With the establishment of the Republic of China in 1912, the republican era was introduced.
The current standard epoch is called "J2000.0" This is defined by international agreement to be equivalent to: The time kept internally by a computer system is usually expressed as the number of time units that have elapsed since a specified epoch, which is nearly always specified as midnight Universal Time on some particular date.
Over time, inexactitudes and other errors accumulate, creating more and greater errors of prediction, so ephemeris factors need to be recalculated from time to time, and that requires a new epoch to be defined.
Different astronomers or groups of astronomers used to define epochs to suit themselves, but in these days of speedy communications, the epochs are generally defined in an international agreement, so astronomers worldwide can collaborate more effectively.
Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era.
The whole purpose and criteria of such definitions are to clarify and co-ordinate scholarship about a period, at times, across disciplines.