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A traveler visiting an Islamic country is greeted by the call to prayer (Adhan) echoing from the tower (minaret) of a nearby mosque. The profound impression of the dawn call ("Prayer is better than sleep, God is great...") has been frequently written about. Let us take a brief look at the calendars used in the Islamic world, starting from the day in terms of Islamic religious practice, and then in the order of week, month, and year. We shall also look at the issue of calendars from the periphery, unrelated to religion.

Islamic prayer (Salat) is performed five times a day—dawn, noon, afternoon, sunset, and night—but in the Middle East, there is a custom that a "day" begins at sunset and ends at sunset. In other words, "Friday" of a given week begins at sunset on Thursday and ends at sunset the following Friday. Because the length of daylight changes every day, the times for prayer must be precisely determined daily. Before the modern era, major mosques in a region had timekeepers who determined prayer times by observing the movement of the sun. Following Babylonian and Egyptian traditions, a week consists of seven days, the same as in Judaism and Christianity. While Saturday is the day of communal worship in Judaism and Sunday in Christianity, Friday was designated in Islam, and many countries still officially observe it as the weekly day of rest.

Regarding months and years, the Islamic religious calendar is a lunar calendar (approx. 354.37 days per year) based on the cycle of the moon's phases (approx. 29.53 days). It begins on the first day of the first month (Muharram) of the year (July 15, 622 AD) when the Prophet Muhammad (c. 570–632) performed the "Hijra" (migration) from his hometown of Mecca to the new land of Yathrib (later Medina [City of the Prophet]). This is known as the Hijri lunar calendar, where each month consists of 29 days for even months and 30 days for odd months, with one day added to the final month in 11 out of every 30 years. Naturally, this calendar creates a discrepancy with the solar calendar (approx. 365.25 days per year) based on the Earth's revolution around the sun. Consequently, the famous month of Ramadan, the "fasting month" during which one abstains from food and drink from dawn to sunset, may fall in winter when days are short, or in summer when the days are long and the heat is grueling. Incidentally, the beginning of a month is defined as the moment a new crescent moon is confirmed by the naked eye in the evening sky after sunset, so the end and beginning of a month may shift by one day from the date determined by calculation. It is tough if the month of Ramadan does not end and requires one more day of fasting, but the sense of accomplishment upon completion is all the greater.

However, a lunar calendar is extremely difficult to use for agricultural activities and for collecting taxes based on harvests. Therefore, in Japan and China, systems such as the lunisolar calendar were used, incorporating devices like leap months every two or three years to ensure the calendar did not drift significantly from the seasons. In the Arabian Peninsula, there was also a lunisolar calendar with leap months before Islam, but it had already fallen into disuse by the time of the Prophet. One theory attributes the cause of this to a revelation in the Quran (Chapter 9, Verses 36–37) condemning the alteration of the twelve months ordained by God, but there are alternative theories, and a definitive conclusion is hard to reach.

Eventually, the Hijri lunar calendar came to be used alongside solar calendars that were dominant in the regions conquered by Arab Muslims from the mid-7th century onward. These included the Julian calendar of the former Roman Empire and the Iranian calendar centered on the spring and autumn equinoxes since the Sasanian Empire (224–651), used to coordinate agriculture and tax collection periods. The financial calendar of the Ottoman Empire was also based on the Julian calendar. In Egypt, there is an agricultural calendar called the Coptic calendar, which improved upon the ancient local calendar (consisting of 12 months of 30 days each with 5 days added at the end for a 365-day year) by adding an extra day every four years, aligning the start of the year with the flooding of the Nile on August 29 of the Julian calendar.

The first day of the Iranian calendar year is called Nowruz ("New Day" in Persian) and is linked to the spring equinox. However, common theory suggests that after the fall of the Sasanian Empire, the year was fixed at 365 days or remained inconsistent due to tax collection timing issues. It was the Jalali calendar, created during the reign of Malik-Shah I (r. 1072–92) of the Seljuk Empire, that fixed it to the spring equinox; this calendar has 365 days with a leap year every four or five years. In these Iranian calendars, the names of beautiful angels or holy beings unique to Iran are used for the names of the months. The current official Iranian calendar, the Solar Hijri calendar, was finalized in 1925, and a similar Solar Hijri calendar is used in Afghanistan.

Another interesting feature is the twelve zodiac signs adopted by Turkic and Mongol peoples from China. These were brought to Iran and its surroundings with the Mongol conquests of the 13th century. The twelve animals were adopted in the same order, and even today, Iranian folk calendars include descriptions and fortunes for each year.

Many calendars have been omitted here; the diversity of calendars in the Islamic world increases even further if those of the Indian subcontinent and Southeast Asia are included. Since the 19th century, the Gregorian calendar (the current Western calendar) has been added and is officially adopted in many countries. Thus, while the Islamic world maintained a pure lunar calendar as its religious calendar, many pre-Islamic calendars remained for agricultural and financial purposes. These were further refined, and with the addition of calendars from foreign peoples, a wide variety of systems have been used. This fact speaks to the cultural diversity of the Islamic world and the flexibility of people's thinking.