how to convert php date formats to GMT and vice versa?

Although the gmdate functions are available. If you are using PHP 5.2 or greater, then consider using the DateTime object.

Here's code to switch to GMT

$date = new DateTime();
$date->setTimezone(new DateTimeZone('GMT'));

and back to the default timezone...

$date = new DateTime('2011-01-01', new DateTimeZone('GMT'));
$date->setTimezone(new DateTimeZone(date_default_timezone_get()));

Using the DateTime object lets your create a datetime, just like the procedural functions, except that you keep a reference to an instance.

e.g.

// Get a reference to Christmas of 2011, at lunch time.
$date = new DateTime('2011-12-25 13:00:00');

// Print the date for people to see, in whatever format we specify.
echo $date->format('D jS M y');

// Change the timezone to GMT.
$date->setTimezone(new DateTimeZone('GMT'));

// Now print the date/time it would in the GMT timezone
// as opposed to the default timezone it was created with.
echo $date->format('Y-m-d H:i:s');

// Just to show of some more, get the previous Sunday
$date->modify('previous Sunday');

There's a whole lot of functions you can use, that are much more readable that the procedural functions.

Explicit example of converting from a timezone to GMT

$melbourne = new DateTimeZone('Australia/Melbourne');
$gmt = new DateTimeZone('GMT');

$date = new DateTime('2011-12-25 00:00:00', $melbourne);
$date->setTimezone($gmt);
echo $date->format('Y-m-d H:i:s');
// Output: 2011-12-24 13:00:00
// At midnight on Christmas eve in Melbourne it will be 1pm on Christmas Eve GMT.

echo '<br/>';

// Convert it back to Australia/Melbourne
$date->setTimezone($melbourne);
echo $date->format('Y-m-d H:i:s');

Using your Asia/Kolkata to America/New_York

date_default_timezone_set('Asia/Kolkata');
$date = new DateTime('2011-03-28 13:00:00');
$date->setTimezone(new DateTimeZone('America/New_York'));
echo $date->format("Y-m-d H:i:s");
//Outputs: 2011-03-28 03:30:00

Use the gmdate function to convert to GMT time.

For example

$d = '2011-03-28 12:05:20'; 
$gmt = gmdate('Y-m-d H:i:s',strtotime($d));