In EnergyPlus, when timestep = 4, the diffuse solar in weather file is different from the output at the same hour. For example, in weather file, at 9:00, the diffuse solar is 58, while in the output, 58 appears at 8:30. This rule seems different from interpolation mentioned in help ref.
In calculating Daylight Factor, EP ref. mentions it uses hourly DF and then interpolate the two hour DF to calculate the DF at time step in one hour. So when calculating Daylight Factor, the Energyplus adopts which hour values of Diffuse and direct Radiation in the Outputfile? Does EP use 58 (at 8:30 in the output), or use value at 8:00 in output (average values at 7:30 and 8:30, which are corresponding to 8: 00 and 9:00 in the weather file respectively) ? .