Jayne and I visit Greece regularly for our hols, so I can tell you that Greece is two hours ahead of GMT. When it's 3 pm in the UK, therefore, it's 5 pm in Greece.
It gets a bit more complicated in the summer, when daylight saving comes into play, and sometimes Greece is then 3 hours ahead of UK time.
The US is at least five hours behind GMT, up to 10 hours in Alaska and Hawaii! So when it's 9 am in New York (for example), by my calculations it would be 4 pm in Greece.
Hope that helps!
Nick
