They (GB FDS) don't know and they never knew, but that didn't stop them from pontificating.
The unrepeatable great tribulation occurred in the first century culminating in the destruction of Jerusalem and the dispersion of survivors throughout the known world...which of course is where the Jews are today and won't happen again.
Needless to say, I believe WT "theologians" as well as others have lifted Matthew 24 and much of the book of Revelation out of its historic context.