By default, we use UTC (Coordinated Universal Time) as a time reference for Solargis time series datasets. UTC is equivalent to GMT (Greenwich Mean Time), therefore UTC+1 equals to GMT+1.
Read more at http://en.wikipedia.org/wiki/Coordinated_Universal_Time
In the case of hourly time series data and TMY data files, we also deliver the data with time reference set as longitudinal time zone, calculated using longitudinal reference of the site as follows: longitude°/15 (rounded value). Please note that in many regions the time zone based on the longitudinal reference will deviate from the legal time (for example in Chile, the time zone used is UTC-3:00, however longitudinally it belongs to UTC-5:00). The only reason for having data in longitudinal time zone is to facilitate easy import of Solargis time series data to PVsyst.
For time series data accessed using Web Services it is possible to specify time zone of your choice.