Parallels of longitude determine the time of a given place.
Local time (of any place) is measured in reference to Greenwich Time (London), which changes at the rate of four min/degree of longitude.
Greenwich Time Line is also known as “Prime Meridian” i.e. 00.
International Standard Time is measured in reference of the Prime Meridian (or Greenwich).
The Earth rotates through 3600 in 24 hours or 150 in 1 hour or 10 in 4 minutes. Therefore, the local time varies at the rate of 4 min/degree of longitude from the Greenwich Time.
As shown in the map given above, Kolkata is approximately 900 east of Greenwich; so, Kolkata is (900 x 4 = 360 min) six hours ahead of the Greenwich Time (London).