Julian day
The months of the pre-Julian Roman calendar are known not to correspond to its namesake Julian day; for example, Livy records a lunar eclipse in 168 BC as occurring on 4 September, when astronomical calculations show it happened on Julian day 21 June of that year. This discrepancy arose from the failure of the pontifices to properly perform intercalations, either by accident or for political advantage. A review of the evidence led P.S. Derow to identify the equivalent Julian date would be 1 July 216 BC; other authorities have suggested other Julian dates.