Indian Standard Time and historical offsets
India uses one time zone, Indian Standard Time (IST), at UTC+5:30 across the whole country. Places far to the east or west have the same clock time, and the sun rises at different clock times in each.
What it is
Historically, several local times were used in India before a standard was adopted. Old birth records may give local mean time for a city rather than IST, and that difference is a few minutes to about half an hour depending on the longitude.
How it is worked out or read
For a birth record in the 20th century, check whether the time is IST or local mean time. Convert to IST before entering a time, or use the longitude difference if the record gives local mean time.
Where it appears on this site
The offset of 5.5 is the default in the tools. The timekeeping article gives the conversion.
The reference meridian
India Standard Time is defined by the meridian of 82 degrees 30 minutes east. That longitude was chosen because it lies close to the middle of the country and gives a time exactly five and a half hours ahead of Greenwich, a convenient offset. The reference point has traditionally been associated with the observatory at Allahabad, now Prayagraj, and the meridian passes near Mirzapur. All official clocks in India keep this time, and it has been the single national time since the late 1940s.
Neighbouring countries
For births near India's borders, or in neighbouring countries, it is worth knowing the nearby zones. Sri Lanka uses UTC plus 5:30, the same as India. Nepal uses UTC plus 5:45, a quarter-hour ahead. Bangladesh uses UTC plus 6, Pakistan UTC plus 5, Bhutan UTC plus 6 and Myanmar UTC plus 6:30. Some of these countries have changed their offsets over the years, so for older births the offset in force at the time should be checked.
Local customs within India
Although IST is the only legal time, some communities have kept informal local times. Tea gardens in Assam, for example, have long kept a 'garden time' about an hour ahead of IST to make better use of the early sunrise. Birth times recorded under such informal clocks are rare but possible, and if a family mentions a local time, it is worth asking how it relates to IST.
Wartime and the old city times
From 1942 to 1945, during the Second World War, clocks in India were advanced by one hour, so the effective offset was UTC plus 6:30. Before IST was fully adopted, Calcutta kept its own time, about 5 hours 54 minutes ahead of Greenwich, and Bombay kept its own, about 4 hours 51 minutes ahead. A birth record from Calcutta before about 1948 or from Bombay before about 1955 may use one of these local times.
What to enter in the tool
For a birth anywhere in India after the mid-1950s, enter an offset of 5.5. For wartime births, use 6.5 unless the record says otherwise. For the old city times, use the local offset if the family's records confirm it was used. In every case, the longitude and latitude should be those of the actual birthplace, since the time zone tells the tool the moment and the coordinates tell it the sky.