We're a few centuries too early for Common/Dionysian dating, so we're relying on several methods of dating used during the Diadokhoi period:
1. The Olympiad dating system, based around the four-year periods between Olympic Games, which have run uninterrupted since, well... The 1st year of the 1st Olympiad, or our 776 BCE. 221 BCE, when our scion comes of age, is therefore the 3rd year of the 139th Olympiad: 780-(139x4) = 224, with the 140th commencing midway thru our year 220 BCE.
2. Regnal years, pretty self-explanatory. Year X of the reign of King blah. 221 BCE marks the first year of the reign of Ptolemaios IV Philopator, while 222 BCE would've been the 24th-ish year of Ptolemaios III Euergetes.
3. Years of the eponymous High Priests of Alexandros -- this is the most widely-used system in the historic Ptolemaic Kingdom. It's a bit of a headache since High Priests' terms rarely extended beyond their year,
but thankfully Wikipedia's come through for us.