(Fatwa-7) Darul Ifta
Jaami'ah Nazeeriyyah Kaakosi, Palanpoor

According to Shari'ah it is not permissible to accept the Jewish or Christian Lunar calendar (which is based on calculations of astronomy). It is also not permissible to
use it as a guide if it results in rejecting of the statutory texts of Shari'ah (Nass).

Mufti Rashid Ahmad Ludhyanwi Saahib has written in one of his Fatawaa:

'According to Shari'ah it is not permissible to use the calculations of astronomy to determine the new moon. The whole Muslim Ummah is unanimous on this matter.'
(Fathul Baari commentary on Saheeh Bukhari Vol.4/Pg.109 and Umdatul Qaari Vol.10/Pg.287)

Read more here.... (pdf) - The Moon Sighting Monthly view inline