2012-11-09T00:00:00+01:00

2012-11-08T23:00:00Z

// Your input String (with no colons in the timezone portion)
String original = '2012-11-09T00:00:00+0100'

// The format to read this input String
def inFormat = new java.text.SimpleDateFormat( "yyyy-MM-dd'T'HH:mm:ssZ" )

// The format we want to output
def outFormat = new java.text.SimpleDateFormat( "yyyy-MM-dd'T'HH:mm:ss'Z'" )
// Set the timezone for the output
outFormat.timeZone = java.util.TimeZone.getTimeZone( 'GMT' )

// Then parse the original String, and format the resultant
// Date back into a new String
String result = outFormat.format( inFormat.parse( original ) )

// Check it's what we wanted
assert result == '2012-11-08T23:00:00Z'

// Your input String
String original = '2012-11-09T00:00:00+01:00'

// The format to read this input String (using the X
// placeholder for ISO time difference)
def inFormat = new java.text.SimpleDateFormat( "yyyy-MM-dd'T'HH:mm:ssX" )