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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 Rome. 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 four times — GMT and Chinese, Vietnamese and Japanese times:
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 (assuming it is being used in England) 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 (as used in the GMT timezone).
Chinese Calendrics allows you to specify a prime meridian (a.k.a. a reference longitude) by selecting a country. The prime meridian of the Vietnamese Calendar is 105° E, that of the Chinese Calendar is 120° E, and that of the Japanese (and the Korean) Calendar is 135° E.
When the Vietnamese Calendar is selected, times are designated as 'ICT' (Indo-China Time), which is GMT + 7 hours. When the Japanese Calendar is selected, times are designated as 'JST' (Japan Standard Time), which is GMT + 9 hours.
Changing the prime meridian by just a few degrees affects the calendar. For example, consider the set of 91 leap months from 1805 CE to 2050 CE calculated using a prime meridian of 120° E. If we were to use a prime meridian of 116° E (although the program does not allow us to choose this) we would find that the leap months are the same except in two cases. With 120° there is a leap month 3* in 1822 and a leap month 12* in 1890, whereas with 116° both 1822 and 1890 have a leap month 2*. The prime meridians of the Vietnamese and Japanese Calendars each differ from the Chinese by 15°, so such differences in months, while not common, are also not rare.
Clicking on the 'Now' button causes the software to read the date and time from your PC's clock (this time is not displayed) and to display this date and time in the various calendars, and to calculate and display the corresponding phase of the Moon.
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