The five-gah cycle is one of the ancient world's clearest systems of fixed daily prayer, and its shape rewards comparison. Havan runs from sunrise to midday; Rapithwin holds the ideal noon; Uzerin carries the afternoon to sunset; Aiwisruthrem the evening to midnight; Ushahin the deep night to dawn. Each watch has its Avestan prayer, its presiding beings and its proper works, and together they mean that Zoroastrian time is never unowned: someone in the hierarchy of the good creation is always on watch, and the believer joins them by prayer.
Scholars of religion have long noted the family resemblance to the five daily prayers of Islam, which arose a millennium later in a neighbouring culture area, and to the canonical hours of Christianity; the traditions are distinct and the lines of influence debated, but the underlying instinct, that piety should be distributed across the day's natural divisions rather than concentrated in one act, is the same, and the Zoroastrian version is among the oldest continuously practised forms of it. The gahs also structure the liturgical rest of the religion: every high ceremony is assigned to its proper watch, the Yasna and Visperad to Havan, the great nocturnal Vendidad service to Ushahin, and the calendar's feasts are celebrated watch by watch. For the Zoroastrian diaspora today the gah prayers remain the portable core of practice, requiring no temple and no priest, only clean ground, the kusti, and the memorised Avestan that has ordered Iranian mornings for well over two and a half millennia.