[rrd-users] RRD Timestamps

[Jonathan Daugherty]

Greetings,

I'm having trouble understanding exactly how RRD is handling
timestamps that I use to render graphs.

I create the RRDs and populate them with data, using local timestamps.
When I run rrdtool to do this, I do not specify a TZ environment
variable value.

I generate graphs from date X, midnight, to date Y, midnight.  I also
include on these graphs two VRULEs at midnight on particular dates.
The problem is that I cannot get the VRULEs and the data points to be
properly aligned; the VRULEs and data points are offset from midnight
by what appears to be the same amount of time as my UTC offset (I'm on
the US west coast).

When I run rrdtool to graph this data and do NOT specify a TZ
environment variable, I get this graph:

  http://www.cprogrammer.org/pub/proper_vrule_skewed_data.png

The VRULEs and beginning and end of graph are correct, all being on
midnight of the specified dates.  However, all of the data points are
shifted by my offset.

If I run rrdtool to graph the data but specify TZ=PDT in the
environment, I get this graph:

  http://www.cprogrammer.org/pub/pdt_skewed_vrule.png

In this case, all of the data points are aligned properly (i.e., all
are aligned with the days, because that's what they're measuring), but
the VRULEs and beginning and end of graph are all shifted by my
offset.  So both of these graphs are pretty hard to read.

What is going on?  What am I missing?  Any help would be greatly
appreciated!

Thanks,

-- 
  Jonathan Daugherty
  JanRain, Inc.
  irc.freenode.net: cygnus in #openid
  cygnus.myopenid.com