The Tenpō calendar (天保暦 Tenpō-reki), officially the Tenpō sexagenary unitary calendar (天保壬寅元暦 Tenpō jin'in genreki), was a Japanese lunisolar calendar (genka reki). It was published in the Tenpō era (1830–1844). It was in use in the late Edo period, from 1844 to 1872.
The calendar is a lunisolar calendar which adopted Teiki-hou for solar terms, in which solar terms are equally divided by solar longitude, in place of Heiki-hou in which solar terms are equally divided by time. The month is a lunar month and the first day of the month starts on the day which includes the new moon. A leap month is added if there are three lunar months between a lunar month which includes a solstice/equinox and the lunar month which includes the following equinox/solstice. In that case, the leap month is the lunar month which doesn't include any Chuuki (one of the twelve solar terms that are used to determine the months of the year). The months which include a solstice/equinox are respectively fixed to the second, fifth, eighth and eleventh months. The time of the day used in the calendar to determine the dates of solar terms and lunar phases is the one observed at Kyoto. 
In previous calendars, hours were of uniform lengths. In the Tenpō calendar, the length of hours changed depending on the time of year. This made it extremely difficult to make Japanese mechanical clocks.
The Tenpō calendar is no longer officially maintained at all.
The calendar didn't define the months of the year when there is only one lunar month or when there are two months which don't include any Chuuki, between a lunar month which includes a solstice/equinox and the lunar month which includes the following equinox/solstice. These problems lead to what is called the Year 2033 Problem.[clarification needed]
- Nussbaum, Louis-Frédéric. (2005). "Calendar" in Japan Encyclopedia, p. 98.
- Orchiston, Wayne et al. (2011). Highlighting the History of Astronomy in the Asia-Pacific Region, p. 155.
- Jessica Kennett Cork. The Lunisolar Calendar: A Sociology of Japanese Time.