How to set timezone in gnuplot? - gnuplot

Question

I have a simple gnuplot command file:
....
set xdata time
set timefmt "%s"
set format x "%H:%M"
....
where x - timestamp column.
Result - time in UTC format. Can I change local timezome for x axis ?

Answer 1

Just came across this in the docs today:
The conversion to and from seconds assumes Universal Time (which is the same
as Greenwich Standard Time). There is no provision for changing the time
zone or for daylight savings. If all your data refer to the same time zone
(and are all either daylight or standard) you don't need to worry about these
things. But if the absolute time is crucial for your application, you'll
need to convert to UT yourself.
From help time/date

Answer 2

I just ran across this today. You don't need to change the format, just change the data. If data.txt contains timestamps in UTC, and you want to display them in PDT (-7 hours off from UTC), simply use:
plot 'data.txt' using ($1+(-7*3600)):2
This subtracts 7 hours (in seconds) from each x value.

Answer 3

A bit late to the party, and I don't have enough reputation points to comment, but to add to anthony and hermannk's answer, this is how I implemented it with their information:
in command.gnuplot, assuming that the first field contains the date/time and the second field contains the value for the Y axis:
plot "inputfile" using ($1 - offset):2
from the (bash) command line, when invoking gnuplot:
gnuplot -e "offset=$(date +%s -d '1 Jan 1970')" command.gnuplot
The offset generally will work OK because gnuplot internally interprets date/time as seconds from epoch. I don't know if the form "offset=$(date ...)" is a bash specific thing, but you could always use "offset=`date ...`" instead.
EDIT --
I found this not to work very well when dealing with DST/Summer Time changes. I changed the offset calc to this, and now it works better:
in command.gnuplot, assuming that the first field contains the date/time and the second field contains the value for the Y axis:
plot "inputfile" using ($1 + offset):2
from the (bash) command line, when invoking gnuplot:
gnuplot -e "offset=$(echo "$(date +%z) * 36" | bc)" command.gnuplot
How does this work?
date +%z provides the timezone offset in the form -0400 or similar, which means Local_Time = UTC - 4 hours. If we consider this a number (-400), then the offset, in seconds, is -400 / 100 * 3600 = -400 * 36.
Note that this won't work with timezones that have 30 minutes increments (India?), but I'm sure with some creating math, you can cover that as well.

Answer 4

In addition to previous correct answer, there is how I do this under bash
From version 4.1 of bash, you could avoid using forks to date for this:
printf -v tzoff "%(%z)T" -1
tzoff=${tzoff:0:1}$(( ( 10#${tzoff:1:2}* 3600 + 10#${tzoff:3:2}* 60 ) ))
Some samples:
TZ=Asia/Katmandu printf -v tzoff "%(%z)T" -1
echo $tzoff
+0545
tzoff=${tzoff:0:1}$(( ( 10#${tzoff:1:2}* 3600 + 10#${tzoff:3:2}* 60 ) ))
echo $tzoff
+20700
TZ=America/Chihuahua printf -v tzoff "%(%z)T" -1
echo $tzoff
-0700
tzoff=${tzoff:0:1}$(( ( 10#${tzoff:1:2}* 3600 + 10#${tzoff:3:2}* 60 ) ))
echo $tzoff
-25200
Then alternatively add this $tzoff to values while creating .dat file or by adding them in plot command file.
printf -v plotcmd 'plot \47file.dat\47 using ($1%s):2' $tzoff
echo "$plotcmd"
plot 'file.dat' using ($1-25200):2

Answer 5

The snippet posted here did not work for me, but this seems to do the trick:
TZ=Europe/Amsterdam tzoff=$(printf "%(%z)T")
tzoff=${tzoff:0:1}$(( ( 10#${tzoff:1:2} * 3600 + 10#${tzoff:3:2}* 60 ) ))
echo $tzoff