A calendar is a matter of calculation; those who set it are human and may be right or wrong. Thus, it should not be depended upon to determine the beginning and the end of the times of Salah (Prayer) and Sawm (fasting), as these were mentioned in the Qur'an and the Sunnah; rather, we should depend on the textual evidences. Although these
astronomical calendars may be beneficial to Mu`adhins (callers to Prayer) and Imams (leaders of congregational Prayer) in approximating the times of Salah , they should not be solely depended upon for Sawm and breaking Sawm, as Allah has decreed their times (Part No. 6; Page No. 142) to be from the break of dawn until the evening, and as the Messenger of Allah (peace be upon him) stated,
Start fasting on seeing the new moon (of Ramadan), and break the fast on seeing the new moon (of Shawwal). If the sky is overcast (and you cannot see it), complete the term (thirty days of Sha`ban.)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.