PSM 30min data files

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Do the DNI, GHI, and DHI data in the downloaded PSM files represent mean W/m^2 values over a 30-min interval? If so, is the mean taken over the preceding interval or the following interval? For example, if a value is reported for 10:30am, is it an average taken over the 10:00-10:30am interval or the 10:30-11:00am interval? Or, is it an instantaneous value taken exactly at 10:30am? Also, I assume that the recorded local clock times do not use Daylight Savings Time - is this correct? Thanks!