IST, UTC and local mean time
Three kinds of time appear in the tools: IST (Indian Standard Time), UTC (Coordinated Universal Time) and local mean time. Each is a different reference, and the tools convert between them.
What it is
IST is UTC plus five and a half hours, used across India. Local mean time is the time based on the sun's position at a city's longitude, which differs from IST by four minutes per degree of longitude from 82.5 degrees east.
How it is worked out or read
The tools take the local clock time and the UTC offset you enter, and compute the absolute moment from them. If a record gives local mean time, convert it first, using the longitude difference from 82.5 degrees east.
Where it appears on this site
The offset field is in each tool. The IST article covers the historical offsets.
How India's clock was set
India Standard Time was adopted as the official time of British India on 1 January 1906. It is based on the meridian of 82.5 degrees east, which passes near Mirzapur in Uttar Pradesh, and is five and a half hours ahead of Greenwich. For some decades afterwards, Calcutta Time and Bombay Time continued in local use, and they were finally abandoned around 1948 and 1955 respectively. Since then, IST has been used across the whole country.
What UTC is
Coordinated Universal Time is the international standard from which all time zones are measured. It is kept by atomic clocks, and occasionally adjusted by a leap second to stay within 0.9 seconds of UT1, the time defined by the Earth's actual rotation. For chart and panchang work this tiny difference is irrelevant; what matters is converting local time to UTC correctly by subtracting the zone's offset.
The time used in the equations
Astronomical equations use a uniform time scale, Terrestrial Time, rather than UTC, because the Earth's rotation is slightly irregular. The difference between them, delta T, is currently a little over a minute and is known from observation for past dates. Software such as the library on this site applies it automatically. A reader does not need to supply it, but it explains why historical charts computed with older software can differ slightly from modern ones.
One zone, a wide country
India spans about 29 degrees of longitude, from about 68 degrees east in Gujarat to about 97 degrees east in Arunachal Pradesh. With a single time zone, the Sun rises nearly two hours earlier in the far east than in the far west by the clock. This has led to periodic proposals for a second time zone in the north-east, though none has been adopted nationally. For panchang and chart work it means that local sunrise, not clock time, must anchor the day.
Practical summary
For births in India since the late 1940s, the recorded time is almost always IST, and the offset to enter is 5.5. For births in Calcutta or Bombay before the dates above, or during the wartime clock advance from 1942 to 1945, the family records should be checked to see which time was used. For births elsewhere, use the offset in force at the birthplace on the birth date.