PARCview’s syntax for the relative time field follows the OPC HDA 1.20 Specification, except that the relative times are calculated based on the local machine’s system time.
From the OPC HDA 1.20 Specification, copyright OPC Foundation, all rights reserved:
OPC Historical Data Access
Specification
(Version 1.20)
The time is considered to be a relative time local to the server. This means that all times are given in UTC time, compute from the current time on the server's local clock. The format for the relative time is:
keyword+/-offset+/-offset…
where keyword and offset are as specified in the table below. Whitespace is ignored. The time string must begin with a keyword. Each offset must be preceded by a signed integer that specifies the number and direction of the offset. If the integer preceding the offset is unsigned, the value of the preceding sign is assumed (beginning default sign is positive). The keyword refers to the beginning of the specified time period. DAY means the timestamp at the beginning of the current day (00:00 hours, midnight), MONTH means the timestamp at the beginning of the current month, etc.
For example, “DAY -1D+7H30M” could represent the start time for data request for a daily report beginning at 7:30 in the morning of the current day (DAY = the first timestamp for today, -1D would make it the first timestamp for yesterday, +7H would take it to 7 a.m. yesterday, +30M would make it 7:30 a.m. yesterday (the + on the last term is carried over from the last term).
Similarly, “MO-1D+5h” would be 5 a.m. on the last day of the previous month, “NOW-1H15M” would be an hour and fifteen minutes ago, and “YEAR+3MO” would be the first timestamp of April 1 this year.
Resolving relative timestamps is based upon what Microsoft has done with Excel, thus for various questionable time strings, we have these results:
10-Jan-2001 + 1 MO = 10-Feb-2001
29-Jan-1999 + 1 MO = 28-Feb-1999
31-Mar-2002 + 2 MO = 30-May-2002
29-Feb-2000 + 1 Y = 28-Feb-2001
In handling a gap in the calendar (due to different numbers of days in the month, or in the year), when one is adding or subtracting months or years:
Month: if the answer falls in the gap, it is backed up to the same time of day on the last day of the month.
Year: if the answer falls in the gap (February 29), it is backed up to the same time of day on February 28.
Note that the above does not hold for cases where one is adding or subtracting weeks or days, but only when adding or subtracting months or years, which may have different numbers of days in them.
Note that all keywords and offsets are specified in uppercase.
Keywords
NOW
The current UTC time as calculated on the server.
SECOND
The start of the current second.
MINUTE
The start of the current minute.
HOUR
The start of the current hour.
DAY
The start of the current day.
WEEK
The start of the current week.
MONTH
The start of the current month.
YEAR
The start of the current year.
Offsets
S
Offset from time in seconds.
M
Offset from time in minutes.
H
Offset from time in hours.
D
Offset from time in days.
W
Offset from time in weeks.
MO
Offset from time in months.
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