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"date" Command

This is one of the commands which has a lot of vendor-unique functions because the original "date" doesn't have enough features. But, you may use only the following functions if you care about compatibility.

  • "-u" option (display in "UTC" timezone)
  • "+format" (to define the format for date and time)

Note that the available macro sequences for the format are written in the last paragraph of "Conversion Specifications" on the POSIX man page of "date." You should make sure that.

To Do the Mutual Transformation

Unfortunately, the original "date" doesn't support the mutual transformation between the usual time and the UNIX time.[1] So, we made the command "utconv" to do it. Of course, it is in POSIX compliant shell script.

Example

What time in UNIX time is for 2015-08-16 10:00:00 JST?

$ export TZ=JST-9
$ echo 20150816100000 | utconv
1439686800
$

And, can the command invert the UNIX time to the original one correctly?

$ export TZ=JST-9 # <- Unnecessary if already typed
$ echo 1439686800 | utconv -r
20150816100000
$

See the header comment block of the command file for detail.

For your information, the ideas which the command followed are the Fairfield's congruence and one implementation code for gmtime() in the standard C library. The former is convert from the usual time to UNIX time. The latter is for the opposite.

  1. Unix time, also known as "Epoch time," is a monotonic number of seconds from 1970-01-01 (UTC).