How these values are computed
Positions of the Sun and Moon come from the NASA/JPL DE440s planetary ephemeris (public domain) read with the Skyfield library (MIT). Apparent geocentric longitudes on the true ecliptic of date are used for tithi (Moon minus Sun, 12° each), karana (6°), nakshatra (sidereal Moon, 13°20') and yoga (sidereal Sun plus Moon). The ayanamsa is Lahiri (Chitrapaksha) as defined by the Indian Calendar Reform Committee: 23°15'00.658" on 21 March 1956, carried forward as a fixed direction. Every transition is found by root-finding to half a second; displayed times are rounded to the minute.
Sunrise. The visible sunrise (upper limb, 34' refraction), the convention used by telugucalendar.org and printed Telugu almanacs. Drikpanchang-style calendars quote the Sun's centre on the true horizon without refraction, about three minutes later; end times of tithi and nakshatra are not affected, only the sunrise and the derived rahu kalam by a minute or two.
Months. Amanta (Amavasyanta) reckoning: a month ends with the amavasya and is named for the rasi the Sun occupies at the new moon that begins it. Two new moons in one rasi give an adhika masa. The samvatsara changes at Chaitra Sukla Padyami (Ugadi).
Day parts. Rahu kalam, yamagandam and gulika are eighths of the daylight span by weekday; durmuhurtam uses fifteen muhurtas of day and night; abhijit is the eighth of fifteen daytime muhurtas; varjyam and amrita kalam use the traditional ghati offsets within each nakshatra (amrita = varjyam + 24 ghatis); brahma muhurtam is the two muhurtas before sunrise counted in the night's own muhurtas.
Festivals. Each festival page states its rule (which tithi, which part of the day, how ties and kshaya tithis are resolved). Dates are computed per city with that city's sunrise, dusk, midnight and moonrise, so the diaspora pages can differ from India by a day where the tithi boundary falls between the two. No festival date is copied from India to another city.
Cities. 215 cities in 21 countries, each with its own coordinates and IANA time zone (daylight saving applied from the tz database). A city is published only if the engine finds a real sunrise and sunset on every day of the span; an observance that depends on moonrise is left out for a city on a night with no moonrise instead of being guessed.
Validation. Tithi and nakshatra names and end times were compared with two published panchangams for 13 dates in three time zones (100% name agreement, end times within 2 minutes), then spot-checked in ten more cities from Seattle to Auckland, Stockholm and Johannesburg; the festival rules were compared with a Telugu calendar's 2026 list (95 of 99 computable entries; the rest are documented). See the validation page.
Not yet covered. Muhurtham lists (marriage, gruhapravesam, vahana): their rules (tarabalam, chandrabalam, lagna, panchaka) are not yet verified against a printed almanac, so none are published. Also rasi phalalu, government holiday lists and Islamic/Christian dates.