As said here the default time offset is defined here since the beginning of ROOT.
You can retrieve it with:
root [0] printf("Default time offset = %d\n",(int)gStyle->GetTimeOffset());
Default time offset = 788918400
This value correspond to the beginning of ROOT. It is 1995-01-01. So it is not 0.
When the time offset is not 0, it is used as explained here:
The time axis will spread around the time offset value. Actually it will go from TimeOffset+wmin to TimeOffset+wmax where wmin and wmax are the minimum and maximum values (in seconds) of the axis.
So in that case, by definition, there is no summer or winter time consideration.
When the time offset is 0 the time start at 1970-01-01 (Unix definition) and the summer/winter times apply.
It is difficult to change the time offset default value to 0 as it is equal to 788918400 since the beginning of ROOT.