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How do I convert Unix (epoch) times to human-readable values?

The only sane way to handle time values within a program is to convert them into a linear scale. You can't store "January 17, 2005 at 5:37 PM" in a variable and expect to do anything with it....

Therefore, any competent program is going to use time stamps with semantics such as "the number of seconds since point X". These are called epoch timestamps. If the epoch is January 1, 1970 at midnight UTC, then it's also called a "Unix timestamp", because this is how Unix stores all times (such as file modification times).

Standard Unix, unfortunately, has no tools to work with Unix timestamps. (Ironic, eh?) GNU date, and later BSD date, has a %s extension to generate output in Unix timestamp format:

   # GNU/BSD date
   date +%s    # Prints the current time in Unix format, e.g. 1164128484

This is commonly used in scripts when one requires the interval between two events:

   # POSIX shell, with GNU/BSD date
   start=$(date +%s)
   end=$(date +%s)
   echo "Operation took $(($end - $start)) seconds."

Now, to convert those Unix timestamps back into human-readable values, one needs to use an external tool. One method is to trick GNU date using:

   # GNU date
   date -d "1970-01-01 UTC + 1164128484 seconds"
   # Prints "Tue Nov 21 12:01:24 EST 2006" in the US/Eastern time zone.

Reading info date (GNU coreutils:Date input formats) reveals that it accepts Unix timestamps prefixed with '@', so:

   # recent GNU date
   $ date -d "@1164128484"
   # Prints "Tue Nov 21 18:01:24 CET 2006" in the central European time zone

However, this feature only works with newer versions of GNU date -- coreutils 5.3.0 and above.

If you don't have GNU date available, you can use Perl:

   perl -le "print scalar localtime 1164128484"
   # Prints "Tue Nov 21 12:01:24 2006"

I used double quotes in these examples so that the time constant could be replaced with a variable reference. See the documentation for date(1) and Perl for details on changing the output format.

Newer versions of Tcl (such as 8.5) have very good support of date and clock functions. See the tclsh man page for usage details. For example:

   echo 'puts [clock format [clock scan "today"]]' | tclsh
   # Prints today's date (the format can be adjusted with parameters to "clock format").
   echo 'puts [clock format [clock scan "fortnight"]]' | tclsh
   # Prints the date two weeks from now.
   echo 'puts [clock format [clock scan "5 years + 6 months ago"]]' | tclsh
   # Five and a half years ago, compensating for leap days and daylight savings time.

A convenient way of calculating seconds elapsed since 'YYYY MM DD HH MM SS' is to use awk.

   echo "2008 02 27 18 50 23" | awk '{print systime() - mktime($0)}'
   # This uses systime() to return the current time in epoch format
   # It then performs mktime() on the input string to return the epoch time of the input string

To make this more human readable, GNU awk (gawk) can be used: The format string is similar to date, and is specifed exactly here at http://www.gnu.org/software/gawk/manual/gawk.html#Time-Functions

   echo "YYYY MM DD HH MM SS" | gawk '{print strftime("%M Minutes, %S Seconds",systime() - mktime($0))}'
   # The gawk-specific strftime() function converts the difference into a human readable format

2012-07-01 04:05