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.
