I think you misunderstand. My point is that the civilization did not collapse, classical learning was not lost, and the old order persisted in the Empire--not the 'city of Byzantium', which by the way was called Constantinople (indeed was still called that down until Ataturk changed the name to the usual way Turks refered to it with the corruption of the Greek phrase eis politas as Istanbul)--not that where wasn't trouble on the borders, a plague, and bad harvests. BTW, do you have some reason for not giving the Imperial capital its proper name?
Of course all A.D. dates have Denys 'initial condition error'. But how is that relevant? The Empire still used the indiction system well into the period when the West used the A.D. system--as I pointed out Anna Comnena writing around 1100 A.D. uses it. What possible relevance does Denys error or when his system was generally adopted have to your point? Are you trying to claim Belisarius retook Ravenna earlier than the standard account so that the mosaics of San Vitale aren't counterexamples to your 'no significant art' claims?
With the environmental catastrophe that precipitated the Dark Ages having happened in 538, a whole lot of what is claimed to have happened between 538 and 541 is also questionable, with some variance given for latitude and other local conditions.
With the Germans having just taken over the Western Empire a few years before the beginning of the Dark Ages, the absence of documentation in the West for that period is frequently attributed to the supposed barbarous nature of those Germans. The supposed survival of Byzantium in a pristine state is further used to underscore the comparative barbarity of the new Roman ruling class.
The fact is they simply lived further North and West and were hit harder than those who lived at the Eastern end of the Mediterranean.
That does not mean Byzantium did not have it's own troubles at the time, which it did. Citing dates at the nexus of the beginning of the collapse as evidence that "nothing happened in Byzantium" is ridiculous.
BTW, it was Byzantium for many centuries before anyone thought to call it Constantinople and I bet you that even in 538 AD there were people living in the "old city" who continued to refer to Byzantium.