[rrd-developers] thinking about localtime and graphing

Tobias Oetiker tobi at oetiker.ch
Fri Nov 25 18:54:39 CET 2011

Hi Alex,

Today Alex van den Bogaerdt wrote:

> > DEF:weekly=test.rrd:ifHCInOctets:AVERAGE:step=3600:ctrigger=%V:tz=CET
> >
> > (%V would be the ISO week number,  but you could use any expression you want).
> >
> > Whenever the value of the ctrigger changes, one 'grouping' comes to
> > an end. If the original data is not available at 'step' resolution
> > from rrd fetch, some interpolation might arificially enhance the
> > resolution prior the blocking it again.
> I'm sure you have already thought about daylight saving. Some
> questions which pop up in my brain are:
> I am not aware of such a time zone, but nevertheless you will
> need to have a 100% sure answer to the following question: Is
> there any time zone in the world which applies DST, and where the
> switch to/from DST is happening at midnight? If there is such a
> time zone, to which day does the extra hour (or whatever the
> offset is) belong to? Does strftime do it right? In every
> implementation?

that is the beauty of it ... its all out of my hands and since
strftime gets much wider testing than rrdtool, chances that it
works corectly are pretty high.

> Other questions, which may not be specific to the current case,
> but it may become more important now, and/or result in more cases
> where RRDtool does not do what the user expects it to do:
> Is 00:00-12:00 (or midnight to noon, or whatever range) invalid
> during summer time, when the user specifies time zone CET ? Or
> does CET also cover the last sunday in March, 1am GMT, until the
> last sunday in October, 1am GMT? If it does, is 2011-08-01 03:00
> CET equal to 2011-08-01 02:00 UTC or to 2011-08-01 01:00 UTC ?

all these questions do not matter, since all I do is, take a unix
time stamp and ask strftime to format it for me ... I then look at
the string that results and when it is different from the string
that resulted from the previous sample, then a new group starts,
otherwhise the same group continues ... this should be pretty
robuts in the face of any odd localtime behaviour.
> How to handle at-style time specifications, asking for example
> for 02:00-03:00 on october 30, 2011, european time? Will it be
> just one hour (because you ask CET, not both CET and CEST), or
> will it be two hours?

this has no influance on this task

> Does strftime behave consistently?

in my experiance it does ... we have been using it for formatting
the labels on the x axis all the time ...
> I don't have the answers, but I will look forward to the
> discussion about this topic. HTH Alex

this toppic should be especially interesting to those who do not
live as close to the GMT time zone as I do ...

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Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900

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