GMT stands for Greenwich (pronounced as grenich) Mean Time. It is the 0 meridian passing through Greenwich, England.
Now a days, England is on BST (British Summer Time) until 25 October 2009. BST is 1 hour ahead of GMT.
How about India? India follows IST which is Indian Standard time. IST is GMT + 5:30 = BST + 4:30
How about US? US has 6 time zones out of which 4 are shared with Canada : EST, CST, PST, MST and the remaining 2 – Alaska and Hawaii Time zones are separate.
Lets look at EST (Eastern Standard Time). EST is normally GMT – 5 . But during Day time Saving ( 2nd Sunday of March – 2nd Sunday of November 2 am). EST = GMT -4
During day time saving in US,
Thus to calculate current time in London, from US, one needs to just add 5 hours to US EST. (4 hours to GMT + 1 hour to BST) .
To calculate Current time in India, from US, one needs to just add (4 hours + 5.5 hours = 9.5 hours)
When day time saving is off,
Current time in London will be 5 hours ahead of US EST (because by that time BST = GMT)
And Current time in India will be 5+ 5.5. hours ahead of US EST = 10.5 hours + EST
For example, if in India time during day time saving is 12 in night,
US EST (day time saving is) : 2:30 in afternoon
US EST ( no day time saving) : 1:30 in the afternoon.
