Vietnamese calendar calculation

Example: lunar calendar for 1984
Let us apply the above rules to determine the lunar dates of the Vietnamese calendar for the year 1984 based on the following table.

Event	GMT time	Hanoi time	Beijing time
New Moon	04/12/1983 12:25:49	04/12/1983	04/12/1983
Principal Term	22/12/1983 10:29:41	22/12/1983	22/12/1983
New Moon	03/01/1984 05:15:34	03/01/1984	03/01/1984
Principal Term	20/01/1984 21:04:50	21/01/1984	21/01/1984
New Moon	01/02/1984 23:46:20	02/02/1984	02/02/1984
Principal Term	19/02/1984 11:15:53	19/02/1984	19/02/1984
New Moon	02/03/1984 18:30:51	03/03/1984	03/03/1984
Principal Term	20/03/1984 10:23:48	20/03/1984	20/03/1984
New Moon	01/04/1984 12:09:34	01/04/1984	01/04/1984
Principal Term	19/04/1984 21:37:38	20/04/1984	20/04/1984
New Moon	01/05/1984 03:45:21	01/05/1984	01/05/1984
Principal Term	20/05/1984 20:57:28	21/05/1984	21/05/1984
New Moon	30/05/1984 16:47:52	30/05/1984	31/05/1984
Principal Term	21/06/1984 05:02:21	21/06/1984	21/06/1984
New Moon	29/06/1984 03:18:24	29/06/1984	29/06/1984
Principal Term	22/07/1984 15:58:22	22/07/1984	22/07/1984
New Moon	28/07/1984 11:51:08	28/07/1984	28/07/1984
Principal Term	22/08/1984 23:00:20	23/08/1984	23/08/1984
New Moon	26/08/1984 19:25:21	27/08/1984	27/08/1984
Principal Term	22/09/1984 20:32:55	23/09/1984	23/09/1984
New Moon	25/09/1984 03:10:31	25/09/1984	25/09/1984
Principal Term	23/10/1984 05:45:27	23/10/1984	23/10/1984
New Moon	24/10/1984 12:08:13	24/10/1984	24/10/1984
Principal Term	22/11/1984 03:10:24	22/11/1984	22/11/1984
New Moon	22/11/1984 22:56:45	23/11/1984	23/11/1984
Principal Term	21/12/1984 16:22:42	21/12/1984	22/12/1984
New Moon	22/12/1984 11:46:36	22/12/1984	22/12/1984

According to the rules, month 11 of a lunar year begins on the day of the New Moon before or on the day of the Winter Solstice and ends on the day before the first New Moon after it. So, month 11 of the lunar year 1983 last from 04/12/1983 to 02/01/1984 and month 11 of the lunar year 1984 last from 23/11/1984 to 21/12/1984.
Between 03/01/1984 (begin of the first month after lunar month 11 of 1983) and 21/12/1984 (end of lunar month 11/1984) there are 12 lunar months. Therefore, 1984 is not a leap year of the Vietnamese calendar, and the month names are straightforward: month 12 from 03/01/1984 to 01/02/1984, month 1 from 02/02/1984 to 02/03/1984 etc.

Why 1985 has a leap lunar month?
The situation is different in 1985. Month 11 of the lunar year 1985 last from 12/12/1985 to 09/01/1986. Between the months 11 of 1984 (from 23/11/1984 to 21/12/1984) and 1985 there are 13 lunar months, so 1985 is a leap year. The month from 21/03/1985 to 19/04/1985 does not contain a Principal Term and is therefore the leap month.