need to split "date" command ouput in shell - linux

Question

I want to split date command output and extract time zone difference and represent that time difference in terms of 30 min.
I tried this:
date -R | awk 'NF>1{print $NF}'
I got the output -0530
This means 5 hours and 30 min difference from GMT. Now I want to convert this -0530 in to -11. So what logic should I use in shell command?

Answer 1

date -R | awk '{tz=$NF;ew=substr(tz,1,1);h=substr(tz,1,3)*2;m=ew substr(tz,4,2)/30;print h + m}'
https://en.wikipedia.org/wiki/ISO_8601#Time_offsets_from_UTC
Edited to work with omitted sign
date -R | awk '{tz=$NF;l=length(tz);m=substr(tz,l-1,2)/30;h=substr(tz,l-3,2)*2;ew=l>4?ew=substr(tz,1,1):"+";print (ew h)+(ew m)}'

Answer 2

No need to complicate things.
tz=$(date '+%z'); echo "${tz:0:1}$(( 2 * ${tz:1:2} + ${tz:3} ))"

Answer 3

I wouldn't, and instead use a different tool. In the perl module Time::Piece you have tzoffset which returns the current timezone skew in seconds.
Thus:
#!/usr/bin/env perl
use strict;
use warnings;
use Time::Piece;
$ENV{'TZ'}='GMT-6';
my $now = localtime;
#find number of 30m differences;
print $now -> tzoffset / 60 / 30;
You can also use variants of strftime to print time formatted to your liking:
print $now -> strftime("%c");
This can be cut down into a one liner if so desired:
perl -MTime::Piece 'print localtime -> tzoffset / 60 / 30'

Answer 4

With GNU date, you could perform math on time/date values (in UTC):
$ date -uR -d "now"; date -uR -d "now -60 min"
Sun, 27 Dec 2015 20:34:17 +0000
Sun, 27 Dec 2015 19:34:17 +0000
A time of -5:30 is -330 min:
$ date -uR -d "now"; date -uR -d "now -330 min"
Sun, 27 Dec 2015 20:34:55 +0000
Sun, 27 Dec 2015 15:04:55 +0000
That could be further moved by the time zone:
$ TZ=America/New_York date -R -d "now"; TZ=America/New_York date -R -d "now -330 min"
Sun, 27 Dec 2015 15:34:26 -0500
Sun, 27 Dec 2015 10:04:26 -0500
That is the local time (now) subtracting 5 hours and 30 minutes.
However, what is a time zone of -5:30?
I know that India is +5:30 and there are some (very few) places that are +11:00.
But I fail to find any -5:30 and -11:00 seems just odd.