The Thai solar calendar, Suriyakhati (Thai: สุริยคติ) was adopted by King Chulalongkorn (Rama V) in AD 1888 as the Siamese version of the Gregorian calendar. It is the legal calendar in Thailand, though Thai lunar calendar dates continue in use. Years are now counted in the Buddhist Era (BE: พ.ศ. พุทธศักราช póota sàk-gà-râat) that is 543 years greater than the Christian Era ( ค.ศ. คริสต์ศักราช krít sàk-gà-râat). As a convenience, calendars typically include the year AD in both Arabic and Chinese numerals.

Mundane astrology figures prominently in Thai culture, so modern Thai birth certificates include lunar calendar dates, and the appropriate Chinese calendar zodiacal animal year-name for both Thai Hora (โหราศาสตร์ โหราสาต ho-ra-sat) and Chinese astrology. Thai birth certificates record the date, month, year and time of birth, followed by the day of the week, lunar date, and the applicable zodiac animal name. Thai traditionally reckon age by the 12-year animal-cycle names, with the twelfth and sixtieth anniversaries being of special significance; but the official calendar determines age at law.

A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun (or equivalently the apparent position of the sun moving on the celestial sphere).

If the position of the earth in its orbit around the sun is reckoned with respect to the equinox, the point at which the orbit crosses the celestial equator, then its dates accurately indicate the seasons, that is, they are synchronized with the declination of the sun. Such a calendar is called a tropical solar calendar.

The duration of the mean calendar year of such a calendar approximates some form of the tropical year, usually either the mean tropical year or the vernal equinox year.

The following are tropical solar calendars:

Every one of these calendars has a year of 365 days, which is occasionally extended by adding an extra day to form a leap year, a method called "intercalation", the inserted day being "intercalary".

The Zoroastrian calendar is a religious calendar used by adherents of the Zoroastrian faith, and is an approximation of the tropical solar calendar.