The Kitáb-i-Aqdas - The Most Holy Book
Index term: time for Obligatory Prayer, under Sunrise
n5.
at noon and in the morning and the evening
Regarding the definition of the words "morning", "noon" and "evening", at which times the currently binding medium Obligatory Prayer is to be recited, Bahá'u'lláh has stated that these coincide with "sunrise, noon and sunset" (Q&A 83). He specifies that the "allowable times for Obligatory Prayers are from morning till noon, from noon till sunset, and from sunset till two hours thereafter". Further, 'Abdu'l-Bahá has stated that the morning Obligatory Prayer may be said as early as dawn.
The definition of "noon" as the period "from noon till sunset" applies to the recitation of the short Obligatory Prayer as well as the medium one.
q83.
 
Question: Concerning the definition of "morning", "noon" and "evening".
Answer: These are sunrise, noon and sunset. The allowable times for Obligatory Prayers are from morning till noon, from noon till sunset, and from sunset till two hours thereafter. Authority is in the hand of God, the Bearer of the Two Names.