The year 311 BC is the anchor for the beginning of the Seleucid era:
The Seleucid era ("SE") or Anno Graecorum (literally "year of the Greeks" or "Greek year"), sometimes denoted "AG," was a system of numbering years in use by the Seleucid Empire and other countries among the ancient Hellenistic civilizations. It is sometimes referred to as "the dominion of the Seleucidæ," or the Year of Alexander. The era dates from Seleucus I Nicator's re-conquest of Babylon in 312/11 BC after his exile in Ptolemaic Egypt, considered by Seleucus and his court to mark the founding of the Seleucid Empire.
Two different variations of the Seleucid years existed, one where the year started in spring and another where it starts in autumn:
The natives of the empire used the Babylonian calendar, in which the new year falls on 1 Nisanu (3 April in 311 BC), so in this system year 1 of the Seleucid era corresponds roughly to April 311 BC to March 310 BC. This included the inhabitants of Coele-Syria, notably the Jews who call it the Era of Contracts (Hebrew: מניין שטרות, minyan shtarot).
The Macedonian court adopted the Babylonian calendar (substituting the Macedonian month names) but reckoned the new year to be in the autumn (the exact date is unknown). In this system year 1 of the Seleucid era corresponds to the period from autumn 312 BC to summer 311 BC. By the 7th century AD / 10th AG, the west Syrian Christians settled on 1 October-to-30 September. Jews, however, reckon the start of each new Seleucid year with the lunar month Tishri.
Since the book quoted from changes the spelling of the original author around without pattern, the transcript of the inscription reads as:
— Michał Gawlikowski: "Les princes de Palmyre", in: Syria. Revue d' art oriental et d' archéologie, 62 (1985), pp. 251-261. doi
Translated into English as:
[…] (Palmyrene and Greek, found on a column in the Great Colonnade): To (Palmyrene: The statue of) Septimius Odaenathus, son of Haeranes, son of Vaballathus Nasor, the most illustrious exarch4 of the Palmyrenes (Palmyrene: rs ’ dy tdmwr=‘Chief of Palmyra’), Julius Aurelius Ate‘aqab (Palmyrene: was made by t‘qb) son of ‘Ogeilu, son of Zabdibol, son of Moqimu, whose surname is Qora, to his friend, with affection (Palmyrene: to his friend, in his honour, under his presidency). In the month of Nisan, the year 563 (April, 252).
— Michael H. Dodgeon & Samuel N. C. Lieu (eds): "The Roman Eastern Frontier and the Persian Wars (AD 226–363). A Documentary History", Rotledge: London, New York, 1994, p60.