What is "ancient church of the east"

n. An Nestorian Church that split from the Assyrian Church of the East over differences concerning the calendar

The Ancient Church of the East ( ʿĒdtā ʿAttīqtā d'Maḏnəḥā, , Kanīsa al-Mašriq al-qadīma), officially the Ancient Holy Apostolic Catholic Church of the East (ܗܝ ܥܕܬܐ ܣܘܪܝܝܬܐ ܕܐܫܬܬܐܣܬ ܒܫܢܬܐ) distinguished itself from the Assyrian Church of the East in 1964. It is one of the Assyrian churches that claim continuity with the historical Patriarchate of Seleucia-Ctesiphon – the Church of the East, one of the oldest Christian churches in Mesopotamia. The church is headquartered in Baghdad, Iraq. In 1970, Catholicos-Patriarch Mar Addai II Giwargis succeeded Mar Thoma Darmo (1968–1969).