Why does TZ=UTC-8 produce dates that are UTC+8?

Question

The current time in Los Angeles is 18:05. But when I run TZ=UTC-8 date --iso=ns, I get:

2013-12-07T10:05:37,788173835+0800

The date utility tells me that the time is 10:05, and even says that it's reporting it as UTC+8. Why?

Answer 1

The reason is that TZ=UTC-8 is interpreted as a POSIX time zone. In the POSIX timezone format, the 3 letters are the timezone abbreviation (which is arbitrary) and the number is the number of hours the timezone is behind UTC. So UTC-8 means a timezone abbreviated "UTC" that is −8 hours behind the real UTC, or UTC + 8 hours.

(It works that way because Unix was developed in the US, which is behind UTC. This format allows the US timezones to be represented as EST5, CST6, etc.)

You can see that's what's happening by these examples:

$ TZ=UTC-8 date +'%Z %z'
UTC +0800
$ TZ=UTC8 date +'%Z %z'
UTC -0800
$ TZ=FOO-8 date +'%Z %z'
FOO +0800

The ISO -0800 timezone format (ISO 8601) takes the opposite approach, with - indicating the zone is behind UTC, and + indicating the zone is ahead of UTC.

Answer 2

Whenever you specify a timezone in the format of +/-00:00, you are specifying an offset, not the actual timezone. From the GNU libc documentation (which follows the POSIX standard):

The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value. It has syntax like [+|-]hh[:mm[:ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 23, and the minute and seconds between 0 and 59.

This is why it appears to be the reverse of what you expect.

Answer 3

Why?

Because POSIX requires it.

If preceded by a '-', the timezone shall be east of the Prime Meridian; otherwise, it shall be west (which may be indicated by an optional preceding '+' ).

So, this will give time near^[1] Los Angeles (with any 3 letter label for time zone text):

$ TZ=ANY8 date "+%Y-%m-%d %H:%M:%S %Z%z"
2016-04-23 10:47:12 ANY-0800

$ TZ=GMT+8 date "+%Y-%m-%d %H:%M:%S %Z%z"
2016-04-23 10:47:12 GMT-0800

And this should give the time near Shanghai, China or Perth, Australia:

$ TZ=ANY-8 date "+%Y-%m-%d %H:%M:%S %Z%z"
2016-04-24 02:47:12 ANY+0800

$ TZ=CST-8 date "+%Y-%m-%d %H:%M:%S %Z%z"
2016-04-23 02:47:12 CST+0800

^{[1] Near because there may be some DST (Daylight Saving Time) in effect that shift the actual "local time".}

Answer 4

As an alternative method you can use the command zdump to show the current time in other timezones + offsets.

Zdump prints the current time in each zonename named on the command line.

The same rules apply with the timezones; west of the prime meridian being "behind" while to the east being "ahead".

Example

$ zdump PST PST Sat Dec 7 03:25:27 2013 PST

I made this script to show several of the timezones + offsets that we're interested in using zdump and date so we could compare them.

$ cat cmd.bash
#!/bin/bash

printf "\ndate: %s\n\n" "$(date)"

for tz in EST PST PST+8 PST-8 UTC UTC+8 UTC-8; do
  echo "-- timezone $tz"
  printf "zdump: %s\n" "$(zdump $tz)"
  printf "date:         %s\n" "$(TZ=$tz date +'%a %b %d %T %Y - (%Z %z)')"
  echo ""
done

Then when you run it you can see the comparison of zdump to date:

$ ./cmd.bash 

date: Sat Dec  7 02:59:05 EST 2013

-- timezone EST
zdump: EST  Sat Dec  7 02:59:05 2013 EST
date:         Sat Dec 07 02:59:05 2013 - (EST -0500)

-- timezone PST
zdump: PST  Sat Dec  7 07:59:05 2013 PST
date:         Sat Dec 07 07:59:05 2013 - (PST +0000)

-- timezone PST+8
zdump: PST+8  Fri Dec  6 23:59:05 2013 PST
date:         Fri Dec 06 23:59:05 2013 - (PST -0800)

-- timezone PST-8
zdump: PST-8  Sat Dec  7 15:59:05 2013 PST
date:         Sat Dec 07 15:59:05 2013 - (PST +0800)

-- timezone UTC
zdump: UTC  Sat Dec  7 07:59:05 2013 UTC
date:         Sat Dec 07 07:59:05 2013 - (UTC +0000)

-- timezone UTC+8
zdump: UTC+8  Fri Dec  6 23:59:05 2013 UTC
date:         Fri Dec 06 23:59:05 2013 - (UTC -0800)

-- timezone UTC-8
zdump: UTC-8  Sat Dec  7 15:59:05 2013 UTC
date:         Sat Dec 07 15:59:05 2013 - (UTC +0800)