The JW's believe that they as an organization were directly involved with and in some cases actually the target of quite a number of Bible prophecies and prophetic parallels that were realized in the 20th century. Most of these are directly tied to dates
Therefore what did or didn't happen on these dates as well as the accuracy of the dates themselves can't be dismissed as irrelavant because they carry an important doctrinal significance.
For example the JW's believe that Christ returned in the fall of 1914. For the next 3 1/2 years he conducted an inspection of all Christianity. He then rewarded the leadership of the Bible Students (a.k.a. Rutherford & Co.) by appointing them over all his belongings in 1919. The entire F&DS doctrine hangs on this alleged appointment.
If that date is wrong, then it becomes obvious even to JW's that this status is entirely self-proclaimed. In other words, the "slave" simply declared himself to be righteous. In the JW world, this is a serious thing, because a slave that uses a self-proclaimed status to assert authority over his fellow slaves, is by defninition, NOT righteous.