I worked for a company based in London for a few years, and they would always say things like "we'll meet at 2pm GMT" etc - and I had to remind them that in the summer, they are BST, which is GMT+1. It sounds pedantic, but since EDT is GMT-4, but London is generally always 5 hrs ahead (except for the weird misalignment of the daylight savings start/end), the meeting time would not be clear.