When the clock strikes 7 a.m. in Jakarta, it is already 8 a.m. in Makassar. In Jayapura, it is 9 a.m. All three cities are in the same country, yet their clocks show different times.
This is because Indonesia has three time zones: Western Indonesia Time (WIB), Central Indonesia Time (WITA), and Eastern Indonesia Time (WIT). Each zone is one hour apart, with WITA one hour ahead of WIB and WIT one hour ahead of WITA.
The division reflects Indonesia's vast geography, stretching from west to east. In 2024, the Geospatial Information Agency (BIG) recorded 17,380 islands with official names and geographical coordinates. Across this wide archipelago, the sun rises and reaches its highest point in the sky at different times.
However, Indonesia has not always had three time zones. Since the Dutch colonial period, the country has changed its timekeeping system several times, at one point dividing the archipelago into six zones.
The Science Behind Time Zones
To understand how time zones work, we first need to look at how the Earth moves. The Earth rotates on its axis from west to east, completing one full rotation in approximately 24 hours.
Imagine dividing the Earth into 360 equal sections. If 360 degrees are divided by 24 hours, each 15 degrees represents approximately one hour. This is why places located 15 degrees apart in longitude have a time difference of about one hour.
Longitude refers to imaginary lines running from the North Pole to the South Pole. These lines help determine the location of places on Earth and calculate differences in time.
The Indonesian government used this principle to establish three time zones through Presidential Decree No. 41 of 1987. It selected three lines of longitude as reference points: 105° East for WIB, 120° East for WITA, and 135° East for WIT.
Since the distance between these reference lines is 15 degrees, each time zone is one hour apart.
WIB covers Sumatra, Java, Madura, West Kalimantan, and Central Kalimantan. WITA includes East Kalimantan, South Kalimantan, Bali, the Nusa Tenggara islands, and Sulawesi. Meanwhile, WIT covers Maluku and Papua.
The decree was issued on 26 November 1987 and took effect on 1 January 1988. The same division remains in use today, as reflected in the official information provided by the Meteorology, Climatology, and Geophysics Agency (BMKG).
How Indonesia Changed Its Time Zones Over the Years
Before adopting the three time zones used today, Indonesia changed its timekeeping system several times. According to a summary by Good News From Indonesia, citing a presentation by the Committee for the Acceleration and Expansion of Indonesia's Economic Development (KP3EI), the archipelago's timekeeping system underwent nine changes during the colonial period and beyond.
The first regulation took effect on 1 May 1908, using Central Java's local time as the reference, at GMT+7 hours and 12 minutes. This meant the time was seven hours and 12 minutes ahead of Greenwich, England, which serves as the world's reference point for time.
Further adjustments followed in 1918 and 1924, including changes affecting Bali, Lombok, Makassar, and Tapanuli.
A major change came on 27 July 1932, when the Dutch East Indies government divided its territory into six time zones, each separated by 30 minutes.
However, this system did not last long. During the Japanese occupation in 1942, Indonesia's time was adjusted to match Tokyo's, at GMT+9, to support military operations. As a result, clocks in Java, which had previously followed GMT+7:30, had to be moved forward.
After Japan's defeat in World War II, the timekeeping system changed again. In 1947, Indonesia was divided into three time zones based on GMT+7, GMT+8, and GMT+9.
However, in 1950, the six-zone system previously used during the Dutch colonial period was reinstated. At the time, Papua, which was still under Dutch administration, followed GMT+9:30.
In 1963, the government once again established three time zones through Presidential Decree No. 243 of 1963. This division became the basis for the three-zone system that was reorganised in 1987. Since then, WIB, WITA, and WIT have remained in place.
Bali and Kalimantan Once Changed Time Zones
In 1987, the government revised the time-zone division because the previous arrangement was considered inconsistent with local geographical conditions and actual local time. The changes particularly affected three regions: West Kalimantan, Central Kalimantan, and Bali.
Previously, West Kalimantan and Central Kalimantan were part of WITA, while Bali followed WIB. Under Presidential Decree No. 41 of 1987, the two Kalimantan provinces were moved to WIB. Bali, meanwhile, was moved to WITA.
The changes took effect on 1 January 1988. The government expected the new arrangement to support smoother government operations and improve work efficiency across different regions.
Indonesia Once Considered Adopting a Single Time Zone
In 2012, the government considered eliminating the country's three time zones and replacing them with a single time zone based on GMT+8.
The proposal was announced in May 2012 and was initially scheduled to take effect on 17 August. However, the date was later postponed to 28 October 2012, coinciding with Indonesia's Youth Pledge Day (Antara, 2012).
One of the main arguments behind the proposal was economic efficiency. The KP3EI Secretariat estimated that adopting a single time zone could save up to IDR 500 billion in trading costs per day. It was also expected to make working hours and financial transactions easier to align with stock exchanges in Singapore, Malaysia, and Hong Kong.
However, the proposal received mixed responses. Hatta Rajasa, Indonesia's Coordinating Minister for Economic Affairs at the time, supported the plan but wanted to hear public feedback first.
Bank Indonesia also recommended giving the public and businesses at least 90 days to prepare for the change.
Former Vice President Jusuf Kalla, on the other hand, opposed the proposal. He warned that the change could disrupt the daily activities of around 200 million people.
He questioned why people should have to adjust their daily routines simply to accommodate stock market trading hours. He also pointed to the United States, which has nine time zones but still operates the world's largest stock market.
The proposed unification was eventually abandoned. Indonesia continues to use three time zones: WIB, WITA, and WIT. So, while someone in Jakarta is just sitting down for breakfast at 7 a.m., their friends or family in Makassar may already be an hour ahead in their morning routine.
