Value represents the interval length in number of seconds. The calculation is performed at evenly spaced intervals
of the specified size. Against a PI 3.x server, the intervals are computed as
evenly spaced UTC time intervals. Against a PI2.x server, the intervals
are computed as evenly spaced wall clock time intervals.
The string is interpreted as a time interval containing an interval count
and the interval type name, for example "1d" or "24h". How the
string is interpreted depends on the PI server type and the interval
type. See the Remarks section below for details. A blank string has the
same meaning as a VT_NULL below.
Calculation interval length is set to the difference between the start
time and the end time specified. This provides a quick way for the
user to specify a single calculation interval.