Sacha Stern's main area of research is the history of time reckoning and calendars in Antiquity and the Middle Ages, which he approaches from the perspectives of the history of science and of social history. He has also researched and published on ancient and medieval Jewish history and early rabbinic literature.

His latest authored book, The Jewish Calendar Controversy of 921/2 CE, is about a controversy that led the Jews across the Near East to celebrate Passover and the other festivals, through two years, on different dates. In this book, he re-edits the controversy texts preserved in the Cairo Genizah, discovers many new Genizah sources, and challenges the historical consensus about the leadership of the controversy, its outcome, and its significance in medieval Near Eastern Jewish history. Through its micro-historical perspective, this book sheds light on the processes that eventually led to the standardization of the medieval Jewish calendar.

His earlier monographs include Calendar and Community: a history of the Jewish Calendar 2nd cent.BCE-10th cent. CE (2001), and Calendars in Antiquity (2012). In the latter, which presents a complete history of the calendars of the ancient world, he argues that the formation of ancient calendars was mostly determined by political factors. He emphasizes the socio-political function of ancient calendars as mechanisms of social and political control, or alternatively as assertions of political independence, sub-culture, and dissidence. Calendars, he contends, were not driven by science but rather by political and social history.

He has directed several major research projects on the Jewish calendar in the Roman Empire and the Middle Ages, including "Medieval Monographs on the Jewish Calendar"(AHRC), "The Jewish Calendar in Early Islamic Sources" (Leverhulme), and "Medieval Christian and Jewish Calendar Texts from England and Franco-Germany" (Leverhulme). In 2013-18, he was Principal Investigator of an ERC (European Research Council) Advanced Grant research project entitled “Calendars in Late Antiquity and the Middle Ages: standardization and fixation”, with a team of five research fellows. In 2018, he led an international research team with Jonathan Ben-Dov (Haifa University) at the Israel Institute of Advanced Studies on 'The Day Unit in Antiquity and the Middle Ages'. In 2018-21, he was Principal Investigator jointly with Ronny Vollandt (LMU, Munich) of a Fritz Thyssen Foundation research project on "Qaraite and Rabbanite Calendars", with Nadia Vidro (UCL) as Research Associate. The same team is now working on a further Fritz Thyssen project on "Saadya Gaon's works on the calendar".

Sacha Stern is editor of the Journal of Jewish Studies (with Alison Salvesen, Oxford University) and of the Brill series Texts and Studies on Time, Astronomy, and Calendars (with Charles Burnett, the Warburg Institute).