To clarify: if you have solar data in the -0:30-0:30 convention you should move the location 7.5 degrees eastward, if you intend using the data in Energyplus. E.g. if your buildings actual location is on the longitude 15 degrees (central Europe) you add 7.5 so that the new longitude parameter (accounting for the time shifting) is 22.5. I tested it with the hyrgothermal simulation tool WUFI which expects solar data in same convention as Energyplus (-1:00-0:00) and has a nice solar radiation rose visualisation.
Joe,
You have an point in that manipulating the longitude is favourable compared to manipulating the data itself. In that way as much information as possible is preserved of the original solar data. I'll start shifting longitude instead of interpolating. And add something like TimeHourOffset=0 or RealLongitude=xx.xx to the epw file's comment field.
Regards,
Lukas