Timezone, UTC offset and daylight saving
The UTC offset is the difference between local time and Coordinated Universal Time. It is what converts a local birth time into the absolute moment used in the calculation.
What it is
A place at UTC+5:30 has a local time five and a half hours ahead of UTC. Some places observe daylight saving time, which adds an hour in summer, so the offset for the date of birth, not the usual one, must be used.
How it is worked out or read
The tools take the offset as a number of hours. If you enter the wrong offset, the time of the calculation moves by the same amount, and the Lagna and Moon may change.
Where it appears on this site
The offset field is in each tool. The timekeeping article explains IST, UTC and local mean time.
What a UTC offset is
Coordinated Universal Time, UTC, is the world's reference time, in practice the same as the Universal Time used in astronomy for chart work. Every local civil time is defined as an offset from it: India Standard Time is UTC plus 5 hours 30 minutes, Nepal is UTC plus 5:45, the United Kingdom in winter is UTC plus 0, and New York in winter is UTC minus 5. To convert a local time to UTC, subtract the offset: 10:00 IST is 04:30 UTC.
Why the calculator needs it
Planetary positions depend only on the absolute moment, and the Lagna depends on that moment and the place. A local clock time on its own does not fix the moment until the offset is known. The birth chart tool on this site asks for the offset directly, in hours, so that it can compute the exact UT moment. Offsets with half or quarter hours are entered as decimals, such as 5.5 for India or 5.75 for Nepal.
Daylight saving
Many countries move their clocks forward by an hour in summer. The offset to enter is the one in force at the moment of birth, which in summer is the daylight saving offset. The rules have changed many times in many countries, so for a birth outside India it is worth checking a reliable historical time-zone reference for the exact date and place. India itself has not used daylight saving in peacetime, but clocks were advanced during the Second World War.
Time zones and longitude
A time zone is a convention; the Sun keeps its own time. Within a wide zone such as IST, the Sun rises earlier in the east than in the west, and local noon falls at different clock times. The calculator uses the offset to find the absolute moment and the longitude to find the local sky, so both must be entered correctly. Entering the wrong longitude with the right offset gives a wrong Lagna, and so does the reverse.
A quick check
After making a chart, check that the Sun's house makes sense for the time of day. Near sunrise the Sun should be close to the Lagna, in the first or twelfth house; around noon near the tenth; around sunset near the seventh; around midnight near the fourth. If the Sun is far from where it should be, the offset or the AM/PM is probably wrong.