A worked example of time-zone conversion
A worked example: a birth at 06:00 local time in India, which is UTC+5:30. The UTC time is 00:30 on the same date.
What it is
Subtract the offset: 06:00 minus 5 hours 30 minutes gives 00:30 UTC. The tool then computes the moment from that UTC time, and the local sidereal time from the longitude.
How it is worked out or read
For a place at UTC+8 with the same clock reading, the UTC time would be 22:00 on the previous day, a difference of over two hours from the Indian case, which would change the Lagna.
Where it appears on this site
Enter the offset in the birth chart tool. See timezone and UTC offset.
Example one: a birth in India
Suppose a person was born on 1 August 1975 at 10:30 in the morning in Mumbai. India Standard Time is five hours thirty minutes ahead of Universal Time, so the birth moment in UT is 10:30 minus 5:30, which is 05:00 UT on 1 August 1975. The calculator needs this UT moment, together with Mumbai's latitude and longitude, to compute the planets and the Lagna. On this site you enter the local time, the place and its UTC offset, and the tool does the conversion; but writing it out once by hand makes clear what the tool is doing.
Example two: a birth with daylight saving
Now suppose a person was born on 15 June 1990 at 8:15 in the morning in New York. In June, New York keeps Eastern Daylight Time, four hours behind UT, rather than Eastern Standard Time, five hours behind. So the UT moment is 8:15 plus 4:00, which is 12:15 UT. If the daylight saving hour were forgotten and five hours added, the result would be 13:15 UT, an hour late, and the Lagna would be off by roughly half a sign or more.
This is the most common time-zone mistake in charts made for births outside India, and a good reason to look up the daylight saving rules in force on the birth date before entering the UTC offset.
Example three: crossing midnight
A birth at 2:00 in the morning on 10 March 2001 in Delhi becomes 20:30 UT on 9 March 2001, because subtracting five and a half hours takes the moment back past midnight. The date in UT is the previous day. Calculators handle this automatically, but a reader checking by hand should remember to change the date, otherwise the Moon's position will be wrong by about thirteen degrees.
Historical cases in India
India Standard Time has been used across the country for most of the period since independence, but there are historical exceptions worth knowing. During the Second World War, from 1942 to 1945, clocks were advanced by an hour, so IST was effectively six hours thirty minutes ahead of UT. Kolkata and Mumbai also kept their own local times for some years after IST was introduced, Calcutta Time until about 1948 and Bombay Time until about 1955. For births in those cities and periods, the recorded time may not be IST, and the family's records should be checked.
A checklist for any birth
For any birth, write down the local time, the place, the time zone that applied on that date, and whether daylight saving was in force. Convert to UT and note the result. Then compare with what the calculator shows. If the chart shows a Lagna or Moon that seems very different from a previously made chart, the time zone is the first thing to check.