Even the humble calendar reveals something of Indonesia’s extraordinary diversity and its turbulent past. Look at this page from an ordinary calendar.
You will notice that under the number representing each date in the month there is other information. You will find the name of the corresponding day in the Javanese five day week (Kliwon, Legi, Pahing, Pon and Wage). To the left and right of this day is the date in the current Javanese month and the current Arab/Islamic month. On the far right you will find the names of these months and the current year in the Javanese and Islamic chronologies.
The last day of the month in the example above is Monday May 31st. or hari Senin tanggal tiga puluh satu Mei. The calendar shows that the same day is also Kliwon the ninth of Besar in the Javanese calendar, and Monday the ninth of Dzulhijjah in the Islamic calendar. For good measure the calendar also indicates that the same day is in the month of Si Gwee in the Chinese Buddhist calendar.
When the world celebrated the arrival of a new millennium on January 1st 2000, an Indonesian calendar would have shown that in the Islamic/Hijriyah era we were in the year 1420, in the Javanese calendar it was 1932, and in the Hindu-Balinese calendar it was 1920.
The Islamic calendar is widely used for religious purposes in Indonesia. It is based on cycles of the moon, not the sun as the European or Gregorian calendar is. The Islamic year is shorter by some 11 days than a year in the international 365-day calendar. Islamic feast days, which are by far the most important holidays for most Indonesians, are celebrated according to the Islamic calendar, so the celebration of them shifts forward by (usually) 11 days each year in the international European/Gregorian calendar. Many millions of Javanese and Balinese also regularly consult their own Javanese and Balinese calendars, especially to keep track of market days and to regulate ritual life.