When everyone stops arguing over timescales and how to measure them, I’ll put more stock in when. Given the constant calendar shifts, it’s the easiest bit to manipulate in more recent history. You could discover a document with a date, forge it, and change the date to have it support or supplant the narrative you’ve got in your sights.
And that’s not even getting into perspective and motivation. We face those problems in empirical studies as it is (subtle manipulations to make something seem true when it is objectively misleading); let alone some long-dead historian who could’ve been offered a bag of gold or his life to write what was desired.