Time Day in Japan Date in the current year: June 10, 2026

The date of Time Day was chosen to commemorate the beginning of official timekeeping in Japan. In the ancient world, water clocks (clepsydras) were the most precise timekeeping instruments because, unlike sundials, they were unaffected by sunlight. It is generally accepted that water clocks emerged independently in several ancient civilizations, including ancient Egypt, Mesopotamia, and China. The Japanese are believed to have borrowed the water clock from China.

The earliest recorded reference to a water clock in Japan is found in the Nihon Shoki (Nihongi), the second oldest book of classical Japanese history. According to the text, a water clock known as a rokoku was installed at the imperial court in Ōtsu in 671 CE, during the reign of Emperor Tenji. It announced the hours with bells and drums. The date of its installation corresponds to June 10 on the Gregorian calendar.

Although the history of timekeeping in Japan is quite long, Time Day itself is a relatively recent invention. It was established in 1920 at the initiative the Tokyo Astronomical Observatory and the Life Improvement alliance, a government organization promoting social reform and modernization, with support from scientific and educational institutions. The goal of the new holiday was not merely to celebrate the anniversary of timekeeping in Japan but also to encourage punctuality, efficiency, and the rational organization of daily life.

Even though it is not an official holiday, Time Day is widely celebrated in Japan by educational institutions and cultural organizations. Schools may teach lessons about the history of clocks, calendars, and astronomy. Museums and science centers often organize exhibitions on timekeeping technology ranging from ancient water clocks to modern precision watches, providing an opportunity to reflect on how societies measure, organize, and value time. The celebration also includes awareness campaigns focused on punctuality and the rational use of time.

One of the most notable events associated with Time Day is the Water Clock Festival (Rōkoku-sai), which is held at the Omi Shrine (Omi Jingu) in Ōtsu, Shiga Prefecture. Built in 1940, the shrine is dedicated to Emperor Tenji, who relocated the Japanese capital from Atsuka to Ōtsu. Since Emperor Tenji introduced the water clock to Japan, the shrine hosts an annual festival commemorating this event.

During the festival, representatives from Japan’s watch and clock industry, dressed in traditional imperial court attire, present new timepieces as offerings and pray for the industry’s continued success. The festival also includes traditional performances and a display of ancient clocks on the shrine grounds. Additionally, the shrine’s Clock Museum and Treasure House are open to the public free of charge on Time Day.