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Midnight in 24-hour clock time is denoted by "00:00" and noon by "12:00". But when it is noon in Beijing it is 04:00 in London (neglecting any differences resulting from the application of summer time) and 05:00 in Oslo. Generally speaking, a time such as 13:05 presupposes a particular longitude such that when it is astronomical midnight at that longitude the clock time is 00:00.
Chinese Calendrics displays three times — GMT, local and a Chinese time:
Time GMT (Greenwich Mean Time) is the time which uses zero degrees longitude (the longitude of Greenwich, England) as its "prime meridian". Timezones correspond more-or-less to regions of longitude. E.g., South-East Asia sets its clocks to GMT + 7 hours because longitude 105 degrees East (105 = 180*7/12) runs through S. E. Asia.
The longitude of Beijing is 116 degrees East, but the timezone of China assumes a longitude of 120 degrees East, since 120 = 180*8/12. Chinese time thus equals GMT + 8 hours. In this software the timezone for China is designated by "CCT", short for "China Coast Time". Thus 20:00 CCT = 12:00 GMT.
Thus a day (or rather, a nychthemeron, a 24-hour period) in the Chinese Calendar runs from midnight CCT to the next midnight CCT. This is in contrast to the Common Era Calendar, where a day (a nychthemeron) runs from midnight GMT to the next midnight GMT. Since the time difference between CCT and GMT is eight hours, days in the Chinese Calendar begin eight hours earlier than days in the Common Era Calendar.
Chinese Calendrics allows you to specify a prime meridian, any value from 100 degrees East to 145 degrees East, and to specify a timezone abbreviation for this prime meridian.
The following are default values:
| 105 °E | ICT | Indochina Time | GMT + 7 hours | |||
| 106 °E | HAN | Hanoi | ||||
| 109 °E | CHA | Chang-An | ||||
| 116 °E | BEI | Beijing | ||||
| 120 °E | CCT | China Coast Time | GMT + 8 hours |
The timezone abbreviations for other longitudes is, e.g., "L23" for longitude 123 °E. The timezone abbreviation for a particular longitude can be changed by editing it.
When the prime meridian is changed the displayed times change, by four minutes for each degree of longitude. E.g., when it is 23:00 BEI with a prime meridian of 116 °E it is 22:32 CHA with a prime meridian of 109 °E, a difference of 4 x 7 = 28 minutes.
Changing the prime meridian by just a few degrees affects the calendar. For example, if we look at the set of 91 leap months from 1805 CE to 2050 CE, first with a prime meridian of 120 degrees East and then with one of 116 degrees East, we find that the leap months are the same except in two cases. With 120 degrees there is a leap month 3* in 1822 and a leap month 12* in 1890. With 116 degrees both 1822 and 1890 have a leap month 2*.
Local time is your local clock time, offset from GMT by a certain number of hours and minutes.
Clicking on the 'Now' button causes the software to read the date and time from your PC's clock (it is assumed that you have set this to your local time) and to display this date in the various calendars and to calculate and display the corresponding phase of the Moon.
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