The Hindu calendar divides the daylight hours of a solar day into five equal parts: prāta - morning saṅgava- forenoon madhyāhna - noon aparāhṇa - afternoon sayāhna - evening These subdivision are used for determining the timing of religious observances. For instance: The tithi revailing at sayāhṇa (not at sunrise) is used to determine when shrardha ceremonies should be performed. Orthodox Hindus perform sandhyāvandana three times: prātahsandhya (at the beginning of prāta), madhyānasandhya (in the middle of madhyāna) and sāyamsandhya (at the end of sayāhna). Since the daylight hours of a solar day varies from day to day (this variation is very visible across seasons), the duration of these five divisions varies from day to day, i.e. by definition these are variable duration calendric elements.