How we calculate
The calculators run in your browser. The birth date, city, and time you type are used to compute a result on that device. They are not sent to CalcJournal and they are not stored.
Astronomy Engine
Planet and horizon positions come from Astronomy Engine, an open source library under the MIT license. The library's stated goal is about one arcminute for dates within several centuries of the present. It is often closer for the Sun. This site does not promise a few arcseconds. A birth time that is off by minutes, or a city that is wrong, moves the result more than the formula does.
Cities and time zones
City search uses GeoNames (CC BY 4.0). The chosen place supplies latitude, longitude, and the time zone, including the daylight saving rule that was in force on that date. The same clock time in two cities is not the same instant.
Signs and cusp days
A zodiac sign here is a 30 degree slice of the ecliptic, counted from the March equinox. It is not the same thing as the constellation with a similar name. The Sun changes sign about once a month. On the changeover day, the year and the clock decide the sign. Date pages show noon UTC for 1992 to 2032 and, on cusp dates, the changeover time in UTC for each year that has one.
What each tool needs
The Sun sign usually follows from the date. The Moon changes sign about every two and a half days, Mercury spends a few weeks in a sign, Venus about a month, and Mars about six or seven weeks. Those tools ask for a time only when the body changes sign that day. The rising sign is the eastern horizon, so it always needs a birth time and a city. It changes about every two hours.
The daily horoscope is a separate note, written for curiosity, not a prediction. It uses the calendar date in Asia/Karachi.