Eight chapters of dated night visions, then six chapters of undated apocalyptic material that reads as if a different writer took over — because most scholars think one did.
What the book is
Zechariah 1 to 8 records a series of night visions received by the prophet, with an angelic interpreter explaining each, together with oracles concerning Zerubbabel, Joshua the high priest and the rebuilding of the temple. Like Haggai, it carries date formulas keyed to the reign of Darius.
Zechariah 9 to 14 has no dates, no visions in the same form, no angelic interpreter and no reference to Zerubbabel, Joshua or the temple project. It consists of oracles in a different register, including material about a coming king on a donkey, thirty pieces of silver, a pierced figure, and a final battle for Jerusalem.
How it is put together
The seam at chapter 9 is one of the most widely accepted divisions in the prophetic corpus. The two halves differ in form, vocabulary, subject matter and outlook, and the second half divides again at chapter 12, each section opening with the same heading formula.
The customary labels are First Zechariah for 1 to 8 and Second Zechariah for 9 to 14, and some add a Third for 12 to 14.
The dating question
The first half is dated by its own formulas to the second and fourth years of Darius, 520 to 518 BCE, and there is no serious dispute about that.
The second half is one of the harder dating problems in the Hebrew Bible. Proposals run from the eighth century BCE to the Hellenistic period. The mention of Greece as a power in chapter 9 has been used to argue for a date after Alexander; against that, Greece appears in Assyrian records as a known entity much earlier. Arguments from the political situation described, from vocabulary, and from relationships to other prophetic books all pull in different directions.
This matters practically. Chapters 9 to 14 are among the most heavily used prophetic passages in later interpretation, and any assessment of a claim built on them has to carry the dating uncertainty with it rather than borrowing the security of the dated first half. Citing a date for Zechariah without saying which Zechariah is one of the commonest quiet errors in popular treatments.
Where the external record touches it
For the first half, the same external framework as Haggai: the reign of Darius I is documented by the Behistun inscription, the Persepolis administrative archives and a wide body of Persian-period material, and the date formulas check out against it.
For the second half there is no external anchor at all, which is exactly why its date is argued from internal evidence and remains open.
The manuscript evidence includes Zechariah material among the Dead Sea Scrolls, in the Minor Prophets scrolls from Qumran and from Wadi Murabba’at, which establishes the text in substantially its present form by the turn of the era.
Related: the overviews of Haggai and Malachi, and the method note on how a text is dated.