Aren't we going to assemble a propaganda events timeline someday? These are some of the most useful features on Wikipedia. 1941, 1973 and 2001 were certainly signal dates in the history of propaganda... you ever hear Salvador Allende's last speech on the radio before Pinochet burst in and had him killed? Hmm... and he was a freedom fighter too.
Wouldn't "signal events" be more appropriate than "signal dates"?
While many events are short lived and have dates, others are of longer duration and still are important events in a timeline of propaganda; for instance the consolidation of timid media; the development of rant radio and TV.
Also, the datable events would be difficult to distinguish which are of signifigance to a timeline, and which are just part of the continuous stream of propaganda and disinformation which persists today.