June 10 is Toki no kinenbi, Time Day. The Tokyo Astronomical Observatory and the Seikatsu Kaizen Dōmeikai, a league for improving daily life linked to the Ministry of Education, established it in 1920.
The date comes from the Nihon Shoki. It records that in 671 Emperor Tenji first announced the hours with a water clock, the rōkoku, and with bells and drums. Converted to the solar calendar, that day is June 10. According to one account, the day was created to call on people to keep time strictly and to make daily life more rational.