Baseline calculations use a series of specific date/time intervals of "like days" to compute the baseline. The EventBaseLineType has xcal:vcalendar and eitc:vcalendar. The eitc:vcalendar contains duration and the evenBaselineDescription with the units, quantity, It is unclear on how to associate the duration with a start datetime.
ENERGYINTEROP-496 and references:
Sources are: http://asset.sce.com/Documents/Business%20-%20Energy%20Management%20Solutions/10DayAvgBaselineFS.pdf
for Enernoc (which cites PJM as using the kW measurements).