Analysis of information sources in references of the Wikipedia article "Dominical letter" in English language version.
The later literary sources describe a period of 12 years without an intercalary day after the reform. This number has always been slightly problematic. Since the reform occurred in 8 B.C., it implies that intercalation resumed in A.D. 5. But A.D. 5 was not a Julian leap year, so the next actual intercalation was in A.D. 8, not 12 but 15 years after the reform. This discrepancy has traditionally been reconciled by interpreting "resumption of intercalation" to mean that accumulation of quarter days started in A.D. 5.
