Christian influences in Islam could be traced back to the Eastern Christianity, which surrounded the origins of Islam. Christians also introduced the Muslims to Greek learning. Eastern Christians (particularly Nestorian Christians) contributed to the Arab Islamic Civilization during the Ummayad and the Abbasid periods by translating works of Greek philosophers to Syriac and afterwards to Arabic. They also excelled in philosophy, science, theology and medicine. The majority of Muslim countries also use a Gregorian calendar and some countries observes Sunday as a non-working day.