According to Hinduism, when does a 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.
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