RESOLUTION INTERVAL

 rrdtool create subdata.rrd -s 10 \
  DS:ds0:GAUGE:5m:0:U \
  RRA:AVERAGE:0.5:5m:300h \
  RRA:AVERAGE:0.5:15m:300h \
  RRA:AVERAGE:0.5:1h:50d \
  RRA:MAX:0.5:1h:50d \
  RRA:AVERAGE:0.5:1d:600d \
  RRA:MAX:0.5:1d:600d

This RRD collects data every 10 seconds and stores its averages over 5 minutes, 15 minutes, 1 hour, and 1 day, as well as the maxima for 1 hour and 1 day.

Consider now that you want to fetch the 15 minute average data for the last hour. You might try

 rrdtool fetch subdata.rrd AVERAGE -r 15m -s -1h

However, this will almost always result in a time series that is NOT in the 15 minute RRA. Therefore, the highest resolution RRA, i.e. 5 minute averages, will be chosen which in this case is not what you want.

Hence, make sure that

1.

both start and end time are a multiple of 900 ("15m")

2.

both start and end time are within the desired RRA

So, if time now is called "t", do

 end time == int(t/900)*900,
 start time == end time - 1hour,
 resolution == 900.

Using the bash shell, this could look be:

 TIME=$(date +%s)
 RRDRES=900
 rrdtool fetch subdata.rrd AVERAGE -r $RRDRES \
    -e $(($TIME/$RRDRES*$RRDRES)) -s e-1h

Or in Perl:

 perl -e '$ctime = time; $rrdres = 900; \
          system "rrdtool fetch subdata.rrd AVERAGE \
                  -r $rrdres -e @{[int($ctime/$rrdres)*$rrdres]} -s e-1h"'

Or using the --align-start flag:

 rrdtool fetch subdata.rrd AVERAGE -a -r 15m -s -1h

AT-STYLE TIME SPECIFICATION

TIME REFERENCE SPECIFICATION

The time-of-day can be specified as HH:MM, HH.MM, or just HH. You can suffix it with am or pm or use 24-hours clock. Some special times of day are understood as well, including midnight (00:00), noon (12:00) and British teatime (16:00).

The day can be specified as month-name day-of-the-month and optional a 2- or 4-digit year number (e.g. March 8 1999). Alternatively, you can use day-of-week-name (e.g. Monday), or one of the words: yesterday, today, tomorrow. You can also specify the day as a full date in several numerical formats, including MM/DD/[YY]YY, DD.MM.[YY]YY, or YYYYMMDD.

NOTE1: this is different from the original at(1) behavior, where a single-number date is interpreted as MMDD[YY]YY.

NOTE2: if you specify the day in this way, the time-of-day is REQUIRED as well.

Finally, you can use the words now, start, end or epoch as your time reference. Now refers to the current moment (and is also the default time reference). Start (end) can be used to specify a time relative to the start (end) time for those tools that use these categories (rrdfetch, rrdgraph) and epoch indicates the *IX epoch (*IX timestamp 0 = 1970-01-01 00:00:00 UTC). epoch is useful to disambiguate between a timestamp value and some forms of abbreviated date/time specifications, because it allows one to use time offset specifications using units, eg. epoch+19711205s unambiguously denotes timestamp 19711205 and not 1971-12-05 00:00:00 UTC.

Month and day of the week names can be used in their naturally abbreviated form (e.g., Dec for December, Sun for Sunday, etc.). The words now, start, end can be abbreviated as n, s, e.

TIME OFFSET SPECIFICATION

NOTE3: If you specify time offset in days, weeks, months, or years, you will end with the time offset that may vary depending on your time reference, because all those time units have no single well defined time interval value (1 year contains either 365 or 366 days, 1 month is 28 to 31 days long, and even 1 day may be not equal to 24 hours twice a year, when DST-related clock adjustments take place). To cope with this, when you use days, weeks, months, or years as your time offset units your time reference date is adjusted accordingly without too much further effort to ensure anything about it (in the hope that mktime(3) will take care of this later). This may lead to some surprising (or even invalid!) results, e.g. 'May 31 -1month' = 'Apr 31' (meaningless) = 'May 1' (after mktime(3) normalization); in the EET timezone '3:30am Mar 29 1999 -1 day' yields '3:30am Mar 28 1999' (Sunday) which is an invalid time/date combination (because of 3am -> 4am DST forward clock adjustment, see the below example).

In contrast, hours, minutes, and seconds are well defined time intervals, and these are guaranteed to always produce time offsets exactly as specified (e.g. for EET timezone, '8:00 Mar 27 1999 +2 days' = '8:00 Mar 29 1999', but since there is 1-hour DST forward clock adjustment that occurs around 3:00 Mar 28 1999, the actual time interval between 8:00 Mar 27 1999 and 8:00 Mar 29 1999 equals 47 hours; on the other hand, '8:00 Mar 27 1999 +48 hours' = '9:00 Mar 29 1999', as expected)

NOTE4: The single-letter abbreviation for both months and minutes is m. To disambiguate them, the parser tries to read your mind :) by applying the following two heuristics:

1.

If m is used in context of (i.e. right after the) years, months, weeks, or days it is assumed to mean months, while in the context of hours, minutes, and seconds it means minutes. (e.g., in -1y6m or +3w1m m is interpreted as months, while in -3h20m or +5s2m m the parser decides for minutes).

2.

Out of context (i.e. right after the + or - sign) the meaning of m is guessed from the number it directly follows. Currently, if the number's absolute value is below 6 it is assumed that m means months, otherwise it is treated as minutes. (e.g., -6m == -6m minutes, while +5m == +5 months)

Final NOTES: Time specification is case-insensitive. Whitespace can be inserted freely or omitted altogether. There are, however, cases when whitespace is required (e.g., 'midnight Thu'). In this case you should either quote the whole phrase to prevent it from being taken apart by your shell or use '_' (underscore) or ',' (comma) which also count as whitespace (e.g., midnight_Thu or midnight,Thu).

TIME SPECIFICATION EXAMPLES

-1month or -1m -- current time of day, only a month before (may yield surprises, see NOTE3 above).

noon yesterday -3hours -- yesterday morning; can also be specified as 9am-1day.

23:59 31.12.1999 -- 1 minute to the year 2000.

12/31/99 11:59pm -- 1 minute to the year 2000 for imperialists.

12am 01/01/01 -- start of the new millennium

end-3weeks or e-3w -- 3 weeks before end time (may be used as start time specification).

start+6hours or s+6h -- 6 hours after start time (may be used as end time specification).

931200300 -- 18:45 (UTC), July 5th, 1999 (yes, seconds since 1970 are valid as well).

19970703 12:45 -- 12:45 July 3th, 1997 (my favorite, and it has even got an ISO number (8601)).