The Kitáb-i-Aqdas: The Most Holy Book (Wilmette: Bahá’í Publishing Trust, 1993, 2009 printing), note 53.
From a letter dated 14 October 2016 written by the Universal House of Justice to the friends gathered in Santiago, Chile, for the dedication of the Mother Temple of South America.
Bahá’u’lláh clarifies, in Questions and Answers, no. 15 of The Kitáb-i-Aqdas: The Most Holy Book, that by “dawn” is meant “the earliest dawn of day, between dawn and sunrise, or even up to two hours after sunrise.”