written by Todd DePastino

Vietnamese soldiers crouching down with rifles on a road during the Tet Offensive

Just after midnight on January 30, 1968, tens of thousands of Viet Cong (VC) and North Vietnamese Army (NVA) regulars attacked US and South Vietnamese forces in II Corps: at Nha Trang, Kon Tum, Hoi An, Tuy Hoa, Da Nang (I Corps), Qui Nhn, Pleiku, and several smaller towns and installations.

It was the start of the famous Tet Offensive, a turning point in the war and in American 20th century history.

But, according to most NVA and VC forces,  the offensive wasn’t supposed to happen until the following day. In fact, insurgents to the south waited until 24-hours later on January 31 to unleash their fury on III Corps and IV Corps, including the capital Saigon.

That difference allowed the US and South Vietnamese to prepare for the attacks they knew were coming.

Nine months in the making and involving almost 100,000 fighters, the Tet Offensive was an enormous operation planned downed to the last detail in order to spark a popular uprising against the US-backed Republic of Vietnam (RVN) in the South and grant the NVA and Viet Cong a decisive victory.

With all the calculation, preparation, and communication that went into the Tet Offensive, how did the VC and NVA screw up something as basic as the date of launch?

In its first postmortem of Tet, the CIA guessed the discrepancy was due to the airtight secrecy of the operation. “The Communists sacrificed coordination for security,” concluded the report, “and this is evident in the premature attacks by units of Military Region 5 (MR 5) on the night of 29-30 January.”

Map of Vietnam with red bursts showing cities targeting during the Tet Offensive

Targeted cities of the Tet Offensive (Images of a Lengthy War. Washington DC: U.S. Army Center of Military History, 1986)

The early assault in MR5, Hanoi’s name for the portion of South Vietnam stretching from just north of Da Nang to Cam Rahn Bay, alerted the South Vietnamese Army and US forces that more attacks were on the way. It was a major error that limited the impact of the uprising.

The mistake was certainly a result of poor communication. But a larger problem was at root.

In 1967, the government of North Vietnam had introduced a new meteorological calendar to calculate the lunar New Year and therefore the holiday of Tết Nguyên Đán – “Festival of the First Day” or Tet, for short.

The change came from Hanoi as a bold challenge to the ancient Chinese calendar the Vietnamese had inherited from their 1,000 years under Chinese domination. Once the Vietnamese achieved independence in 938CE, imperial meteorologists and mathematicians in Hanoi began work on a Vietnamese calendar. The new official Dai Viet (“Great Viet”) administration used the Vietnamese calculations. But most of the population continued with the old Chinese version to which they had been accustomed.

On August 8, 1967, Ho Chi Minh’s Democratic Republic of Vietnam decreed a change in the lunar calendar, perhaps to re-assert its nationalism and independence from China. The new calendar was unique to Vietnam’s geography, whose longitude is over 10 degrees west of Beijing’s. Hanoi’s position also meant it’s time zone was one hour earlier than China’s.

All this added up to a crucial 46-minute discrepancy between the appearance of the new Moon in Hanoi and its rise in Beijing. In Vietnam in 1968, the Moon became completely dark, stationed between the Earth and Sun, at 11:29pm on January 29. In eastern China, the same thing happened at 12:15am on January 30.

For a holiday calendar to be truly Vietnamese, then, the first day of Tet would be celebrated on January 29. But for those adhering to the traditional Chinese calendar—like most of the people in South Vietnam—Tet would be observed on January 30.

The launch of the Tet Offensive was scheduled for the day after the first day of Tet.

Only those closest to the thinking in Hanoi, that is, those in Military Region 5, observed the first day of Tet on January 29. VC cadre further to the south in III Corps and IV Corps, in contrast, followed the patterns of the South Vietnamese peasantry and continued to use the Chinese calendar. For them, the first day of Tet was on January 30.

That slight meteorological distinction, combined with a one-hour time change, gave US and South Vietnamese forces a crucial 24-hours to prepare for the larger attacks on the last day of January.

Today, some anti-Communist Vietnamese ex-patriates in the US and elsewhere detect more sinister intentions behind the admittedly strange calendar decree of 1967.

Members of the Vietnamese National Party, writing on their website, argue that Hanoi Meteorological Department made the calendar change for purely political reasons, not scientific ones. “The intention,” they write, “was probably aimed at the Tet Offensive campaign. . . . after the North Vietnamese people finished celebrating Tet, the Communists attacked the day the Vietnamese people celebrated Tet.”

That may, indeed, have been the intention. But it was not the complete result.

The Tet Offensive was an utter disaster for the North Vietnamese and the VC insurgents in the South. US and South Vietnamese beat back the assault and destroyed enemy forces over the next seven months. By messing with the calendar, Hanoi made its own defeat more likely.