Does the day start at sunrise or 12:00?
For fasting and other reasons when we need to do certain tasks for 1 day. When does the day start?
In KrishnaBhaagwatam, Book 4, Chapter 31
यथैव सूर्यात्प्रभवन्ति वारः पुनश्च तस्मिन् प्रविशन्ति काले ।
भूतानि भूमौ स्थिरजङ्गमानि तथा हरावेव गुणप्रवाहः ॥ १५ ॥
Just like days start from Sun's appearance, and again after some time end within Sun. The same way all the existence, moving or not moving, on living dimension, are like flow of the Gunas(Sattva, Rajas, Tamas) in Hari.
This is a statement in Bhagawatam that says day start with Sun and end with Sun.
Day starts from sunrise. One day and night together makes an ahorAtra. For vrata and other auspicious occasion we have to consider sunrise as the.start of the day.
It is advised to get up about two muhurta before the sunrise in order to finish day to day chorus and get ready for rituals like sandhyA.
One muhurta as per Astrology is about 48 minutes. Let us see a passage from wikipedia to understand it better.
Brahma Muhūrta, approximately one and a half hours before sunrise or more precisely is 1Hr 36 Mins. i.e. 96 Minutes = 2 Muhūrta or 4 Ghaṭīkā, is recommended in all practices of yoga is traditionally considered most apt for meditation.
However, it is clear from the associations of the names with specific constellations that the present Brahma-Muhūrta starts just before 6:00AM during the Vernal Equinox. At present, Jīva-Amṛta and Viṣṇu comprise the two twilight muhūrtas prior to sunrise.
Sunrise and sunset times can be found in local panchAngam and newspapers
It is generally considered by Hindus now-a-days that the day begins with the sun rise. But in Vedic period the day began with dawn (Usha kal) exactly when redness appear in the sky indicating the rise of Sun. This is the period which was called then as Pratah Savan for worship. RigVeda describes three Savans of a day but I could find only two of them so far - (1) Pratah Savan (2) Madhyan Din Savan.
(1) Pratah Savan - it seems that the Pratah Savan was devoted exclusively to our ancient Mother Godess Aditi in earlier age as mentioned in RV. 5.69 (3) - " Pratah Devim Aditi johavimi Madhyan Din udita Suryasya" meaning - " we recall/ remember godess Aditi in the early morning and the rising Sun in the mid day ritual". (here ' the rising Sun may be indicative of Indra). But in another hymn RV.7.41 (1) of the verse addressed to Godess Usha mentions many gods for Pratah Savan - "Pratah Agnim Pratah Indram havamahe ....Pratah Somam ut Rudram huvem".
In order to affirm when this Pratah Savan begins, a reference to a verse addressed to Usha RV.7.77 (1) would suffice, where Godess Usha " shouting/making hassle like young girl awakes every lives of the inhabited world and directs to their respective work" ( Usho ruruche yuvatih na yosha visham jeevam prasuvanti charaye". This time corresponds with the Brahma Muhurt that later scriptures design.
This sloka is from Pancha Siddhantika. Chapter 15. "Secrets of Astronomy".
"19. Simhaacharya has declared that reckoning day-total commences at sunrise in Lanka. "
Hindu Siddhantas suppose Lanka to be on the equator, at the junction of the Ujjain meridian. Simhaacharya's view is that of many later siddhantas.