The beginning of a day according to Islamic calculations

Q 1: When does the day start according to Islamic calculations? Does it start from the break of the dawn, or after midnight as astronomers count? Please, provide evidence from the Qur'an or the Sunnah.

A: In Islam, the day starts at the break of the second true dawn. Allah (Exalted be He) states, It is made lawful for you to have sexual relations with your wives on the night of As-Saum (the fast). Until it reads: So now have sexual relations with them and seek that which Allâh has ordained for you (offspring), and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall. The Prophet (peace be upon him) stated, Bilal pronounces the Adhan at night, so eat and drink (Suhur) until Ibn Umm Maktum pronounces the (second) Adhan. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.