We all have events in history that have taken place that we use for reference points when trying to recall something from the past. For the older generation, the assassination of John F. Kennedy is one of those markers. For others it may be the Twin Towers being flown into, or possibly a more recent event would be Covid. We find Isaiah doing the same thing when making the comment in the year King Ussiah died. That was a marker for them. Isaiah goes on to share how he had a vision of the true King and as a result true reality set in. A Reality Check so to speak...
