: Unification of the Islamic Calendar: A Dialogue on Wujûd al-Hilâl and the Visibility of Hilâl. Within the vocabulary of the Islamic calendar, particularly in Indonesia, the terms al-Hilâl and the sighting or “visibility” of the hilal (imkân al-ru'yah) are recognised. Wujûd al-hilâl is a creative synthesis or half way between ijtimâ' (qabl al-ghurûb) and the theory of the visibility of the hilâl or, in other words, half way between pure hisab (calculating the hilâl) and ru'yah (the sighting of hilâl). Therefore, for the theory of wujûd al-hilâl, the method used in starting the first day and a new month on the Islamic calendar is not merely the process of the occurrence of astronomical conjunctions but also considering the position of hilâl at sunset. Meanwhile, the visibility of hilâl is a construction of theories that which are sourced from the experience of observers. As a result, various typologies are created. In order to unite both concepts, an assertive and continuous dialogue is required.