The date of the most important event in history (according to them) was unimportant?
Dates in general. Note that they also do not give a specific date for Christ's birth or the destruction of the Temple, two other dates of major significance for Jesus's disciples. These were common men, not historians. Since dates later commanded a lot of historical significance, it makes no sense whatsoever that if the Gospels were fabricated, as you claim to believe, the collaborators would not have come up with a date. The fact that no date is provided only adds to the plausibility.