Actual sunset ends the fast. Dark clouds, fading daylight, or an inaccurate clock are not themselves proof that the sun has set.
One should use reliable local time information or clear evidence, rather than treat an unsupported guess as an established sunset.
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Kitab al-Sawm: Mistaken Dawn or Sunset, Qada and Kaffarah
Actual sunset ends the fast. Dark clouds, fading daylight, or an inaccurate clock are not themselves proof that the sun has set.
One should use reliable local time information or clear evidence, rather than treat an unsupported guess as an established sunset.