たとえば2007年にはベトナムでは2月17日を旧暦の1月1日としたが、中国では1日遅い2月18日を旧暦の元旦とした。こうしたずれは数年に一度起こるがほとんどの場合旧暦の翌月1日には解消されるので、旧正月のずれを引き起こす1月以外では大きな混乱を引き起こすことはないとされる。

ベトナムの旧暦は中国暦とほぼ同じであるが、1968年から、ベトナム標準時( UTC+7 )を使うようになった。 台湾、香港、マカオも中国標準時と同じ( UTC+8 )を使用している。東南アジアやアメリカなどの華人社会でも 中国標準時を使用する。

ベトナムの旧暦は中国暦とほぼ同じである。ただしベトナム標準時(UTC+7)を使うため、日付は異なることがある。旧暦の扱いは中国などに似ていて、旧正月(テト)はやはり盛大に祝われる。国外ではベトナム暦とも呼ばれる。

used the Chinese calendar based on the longitude of Beijing (116ı250 East), even

though the longitude of Hanoi is 105ı550 East. However, on August 8, 1967, the

North Vietnamese government approved a new lunar calendar specifically compiled

for the UTC C 7 time zone. The following year, the Chinese New Year new

Moon occurred at 16h 29m UTC on January 29. In North Vietnam, the new Moon

therefore occurred at 23h 29m UTC C 7 on Jan 29. So Chinese New Year, known

as Tet in Vietnam, was celebrated on January 29. However, in South Vietnam, the

new Moon occurred 46 minutes (11:5ı

) later at 0h 15m on January 30, so Tet was

celebrated on January 30. The North Vietnamese Army and the Vietcong guerillas

were preparing for what would be known as the Tet Offensive. The instructions

were to attack in the early morning of the second day of Tet. The units in Da Nang

and other Central Vietnamese cities had closer links to North Vietnam and were

aware of the calendar change, so they attacked on the morning of January 30, the

day after the new Tet, while in Saigon and other cities to the South, everybody was

using the traditional calendar, and the attack started on the morning of January 31,

the day after the traditional Tet.

The Vietnamese calendar is a lunisolar calendar that is mostly based on the Chinese calendar. As Vietnam's official calendar is the Gregorian calendar since 1954,[1] the Vietnamese calendar is used mainly to observe lunisolar holidays and commemorations, such as Tết and Mid-Autumn Festival.

Beginning in 1954, Vietnamese administrative offices officially used the Gregorian calendar, while the civilian populace continued to use a variety of local calendars derived from French, Chinese and Japanese sources, including the Hiệp-kỷ calendar.[1] On August 8, 1967, the North Vietnamese government issued a decree to change Vietnamese standard time from UTC+8 to UTC+7, as well as make the Gregorian calendar the sole official calendar, restricting lunisolar calendar use to holidays and commemorations. South Vietnam would later join this change at the end of the Vietnam War in 1975.