http://www.cogsci.indiana.edu/farg/harr ... etapro.htm
The above link that is mentioned says that after the fourth century BC zeta is pronounce as “z”.
However, at some time in the 4th century BCE the change to the modern Greek pronunciation of z as [z] was already taking place. Aristotle (Metaphysics, 993a) writes that whereas some people would analyze z into s+d, others consider it as a separate sound which does not consist of already recognized elements. At the same time there starts to be some confusion between z and s in Greek inscriptions (e.g., anabazmous instead of anabasmous, 329 BCE).
Further evidence for a later continuous (fricative) pronunciation of z ([z]) comes from ancient Greek grammarians (e.g., Dionysius Thrax), who divide consonants into two primary categories: the aphona (beta, gamma, delta, kappa, pi, tau, theta, phi, and chi), and the hemiphona (zeta, ksi, psi, lambda, mu, nu, rho, sigma). In Aristotle's Poetics (1456b) the aphona are described as "having contact" (= "meta prosboles"), but not being pronounceable without a vowel. In modern parlance we would say that aphona are the plosives, pronounced instantaneously, while hemiphona (of which zeta is a member) are fricatives, and those other consonants that can be pronounced continuously, without the need for a following vowel. This agrees with a pronunciation of z as [z].
Ancient Persian names that contain the consonants [zd] are transliterated in Greek through z. For example, in Plato we have Oromazes (Ωρομάζης) for Persian Auramazda; and in Xenophon we find Artaozos (Αρτάοζος; in Herodotus: Artavazos) for Artavazda. The Hebrew name Ašdod, we find it in Herodotus as Azotos (Άζωτος).
Ancient Greeks were never keen to render the sounds of foreign words correctly, but rather to improve them and make more easier for a Greek to pronounce, in other words to “hellenise”. Therefore this evidence does not count. According to that tradition we have
Phraortes for (Persian) Kshatrita (not even close),
Cyaxares for Uwakshatra (!),
Astyages for Ishtumegu (!),
Cyrus for Kurush,
Cambyses for Kambujiya,
Darius for Darayavahush,
Xerxes for Xshayarsha,
Artaxerxes for Artaxshassa
Arses for Arsha
Hystaspis for Vishtaspa (!).
For Xerxes we have in the Bible Ahasuerus (in the Greek transcription). Also to mention here Jesus for Joshuah, Maria for Myriam and all the biblical names.
Getting back to our “evidence”, the Hebrew word Ašdod to a Greek sounded to much barbaric, and has to be improved to Azotos (zeta as “z”!). This tradition is followed even today, where Shakespeare becomes Sekspirios (used till the 19th century, though today they pronounce it in the English way) and Hegel becomes Egelios.