Clock reset

[swiftflo]

After a recent transatlantic journey(Caribbean etc) my iphone 4 has thrown a wobbly regarding the time.
In the past it has always kept the correct time, even when the clocks are altered to BST and back.
Since my journey I have had to have the clock settings on manual, if I set the setting to Auto the clock is 5 hours slow. Anyone any idea what has gone wrong and how to rectify it.

Thanks.