Vietnamese calendar

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, the Vietnamese calendar is used mainly to observe lunisolar holidays and commemorations, such as Tết and Mid-Autumn Festival.

After Vietnam regained independence following the third Chinese domination of Vietnam, monarchs established their own calendars based on Chinese prototypes, and every subsequent dynasty had appointed officers to man and create the calendar to be used in the realm. Beginning in 1324, the Chinese Yuan dynasty introduced the Thụ thời (Chinese: 授時曆 Shoushi) calendar to the Vietnamese Trần dynasty.

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. On 8 August 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.

The Chinese calendar is based on astronomical observations and is therefore dependent on what is considered the local standard time. North Vietnam switched from UTC+8 to UTC+7 on 8 August 1967, with South Vietnam doing likewise in 1975 at the end of the Vietnam War. As a result of the shift, North and South Vietnam celebrated Tết 1968 on different days. This effect would see the solstice falling on 21 December in Hanoi, while it was 22 December for Beijing.

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