The Book of Ezekiel begins “in the thirtieth year.”
Now it came to pass in the thirtieth year, in the fourth month, in the fifth day of the month, as I was among the captives by the river Chebar that the heavens were opened, and I saw visions of God.
וַיְהִי בִּשְׁלֹשִׁים שָׁנָה, בָּרְבִיעִי בַּחֲמִשָּׁה לַחֹדֶשׁ,
וַאֲנִי בְתוֹךְ-הַגּוֹלָה, עַל-נְהַר-כְּבָר; נִפְתְּחוּ, הַשָּׁמַיִם, וָאֶרְאֶה, מַרְאוֹת אֱלֹהִים.
— Ezekiel, 1:1
— (“old”) JPS trans. (1917), borrowed from Mechon-Mamre
The New Jewish Publication Society translation (1999) notes: “We do not know the 30th of what.”
One common suggestion is that this references Ezekiel’s 30th year. In this context it is useful to know that priests were to take up their duties at the age of 30 (see, e.g., Numbers 4:3) and that Ezekiel was from a priestly family. (Jewish Encyclopedia article; My Jewish Learning)
The 30th-year-of-what question is only the first of many challenges in the Book of Ezekiel. Chapter 1 describes visions that became the source of mysticism known as Maaseh Merkavah “Works of the Chariot.” This is one of the many forms of expertise ascribed to Rabbi Johanan ben Zakkai in the both Sukkah 28a and Baba Bathra 134a, cited on Day 5.