Monday, February 4, 2019

An Off-by-one Error in the Tet Offensive (1968)

North Vietnam used UTC+07:00 since 1968-01-01.
South Vietnam used UTC+08:00 since 1960-01-01.

The relevant new moon was on 1968-01-29, 16:31 UTC,
in North Vietnam time, 1968-01-29, 23:31.
In South Vietnam time, 1968-01-30, 00:31.

In Chinese and Vietnamese calendars, months begin on the day of the new moon. Years begin on the second or third new moon after the winter solstice.

Tet was on 1968-01-29 in North Vietnam.
Tet was on 1968-01-30 in South Vietnam.

The first wave of attack was small, and happened "on the night of 29-30 January" of 1968. Without further details, I would assume that it means it happened at around 1968-01-30 00:00 UTC+7.

The second wave of attack was large, and happened starting at 1968-01-31, 00:00 UTC+7.

Without looking for actual reports of what the battle plans actually were, I'd just guess that the North Vietnam soldiers were ordered to "attack on the midnight of Tet".

That's a lot of trivia, I guess. But it's significant too.
Off by an hour.
Off by a day.
Giving South Vietnam forces a day of warning and time to prepare.
Losing North Vietnam forces the element of surprise.

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