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There are a few known double-dated coins from the reign of Tiberius (e.g. here and here). Both of these coins provide a regnal year for Tiberius and the Actian year; a count from the Battle of Actium. The Battle of Actium is a significant datum in Roman chronology, because we know with great certainty that it occurred in September 31 BC. Pairing the Actian years with the regnal years of Tiberius on these two coins shows that Tiberius' reign was reckoned from after the death of Augustus, regardless of whether or not there was a co-regency.

*Tiberius' reign was probably reckoned from September AD 14 or January AD 15, though I acknowledge there's an outside chance it was reckoned from August AD 28

There are a few known double-dated coins from the reign of Tiberius (e.g. here and here). Both of these coins provide a regnal year for Tiberius and the Actian year; a count from the Battle of Actium. The Battle of Actium is a significant datum in Roman chronology, because we know with great certainty that it occurred in September of 31 BC. Pairing the Actian years with the regnal years of Tiberius on these two coins shows that Tiberius' reign was reckoned from after the death of Augustus, regardless of whether or not there was a co-regency.

*Tiberius' reign was probably reckoned from September AD 14 or January AD 15, though I acknowledge there's an outside chance it was reckoned from August AD 14

A double-header coin may provide supporting evidence for a co-regency...or it may not. Other powerful Roman figures were memorialized on coins without being Emperor (e.g. Tiberius' mother, see here). The trouble is, without a date that can be paired with a known calendar, a double-header coin does not allow us to answer the central question: from when was the reign of Tiberius reckoned?

A double-header coin may provide supporting evidence for a co-regency...or it may not. Other powerful Roman figures were memorialized on coins without being Emperor (e.g. Tiberius' mother, see here). The trouble is, without a date that can be paired with a known calendar, a double-header coin does not allow us to answer the central question: from when was the reign of Tiberius reckoned? (note that even in a co-regency sometimes the reckoning for the 2nd ruler starts from when the 2nd ruler became co-regent, and sometimes it starts from when the 1st ruler died)

It is unclear whether Tiberius' regnal years were reckoned from September or from the following January (Roman New Year's).

Given the degree of fame John held, I suggest the very lower end of that range is quite unlikely.

It is unclear whether Tiberius' regnal years were reckoned from *September or from the following January (Roman New Year's).

Given the degree of fame John held, I suggest the very lower end of that range is quite unlikely.

Appendix - Coregents & Coins

Coregency of Augustus & Tiberius

I have not taken a position on whether there was or was not a co-regency between Augusts & Tiberius, because I do not believe the available evidence allows us to conclusively rule out one or the other possibility. Suetonius appears to support the view that there was a co-regency, Tacitus & Dio Cassius appear to oppose it. But none of them pay much attention to it either way.

The proposal I have outlined above works whichever side of the co-regency debate we land on.

Coins

There is value in distinguishing between double-header coins & double-dated coins. A double-header coin includes the figures of two individuals; a double-dated coin provides the year the coin was minted on two calendars.

A double-header coin may provide supporting evidence for a co-regency...or it may not. Other powerful Roman figures were memorialized on coins without being Emperor (e.g. Tiberius' mother, see here). The trouble is, without a date that can be paired with a known calendar, a double-header coin does not allow us to answer the central question: from when was the reign of Tiberius reckoned?

A double-dated coin is historical gold (pun intended). It provides the year on two different calendars. If one calendar is known, it allows us to understand/utilize the other. It's kind-of like a Rosetta Stone for coins.

There are a few known double-dated coins from the reign of Tiberius (e.g. here and here). Both of these coins provide a regnal year for Tiberius and the Actian year; a count from the Battle of Actium. The Battle of Actium is a significant datum in Roman chronology, because we know with great certainty that it occurred in September 31 BC. Pairing the Actian years with the regnal years of Tiberius on these two coins shows that Tiberius' reign was reckoned from after the death of Augustus, regardless of whether or not there was a co-regency.

As such, I maintain that the ministry of John the Baptist, which commenced in the 15th year of Tiberius, began no earlier than late in AD 28.

Miscellaneous

My channel has a video reviewing the relevant coin arguments here.

For an in-depth discussion of competing numismatic claims, see here.

The argument for dating Tiberius' reign from a co-regency is indeed popular--but it is relevant to consider why it is popular. Far and away the most common motive for dating Tiberius' reign from AD 12 (or 11, or 13) is to develop a chronology that aligns with the Schurer hypothesis for the timing of the death of Herod the Great. Though Schurer's views were politically popular in the 1890s, he had to make several significant assumptions to get the math to work, and his calculations have been repeatedly challenged in more recent years. My own video series on the chronology of Herod discusses this here.

*Tiberius' reign was probably reckoned from September AD 14 or January AD 15, though I acknowledge there's an outside chance it was reckoned from August AD 28