How to set system time from Unix timestamp

The Unix time is given as seconds since the epoch: the number of seconds (not counting leap seconds) that have passed since 00:00:00 Coordinated Universal Time (UTC), or Thursday, January 1st, 1970,

The GNU date command has some very nice features that allow you to translate between different time formats. These are explained very nicely in man date so I will only give you some examples here:

### "Normal" format
$ date
Thu Jun 12 11:44:23 CEST 2014
### Unix time
$ date +%s
1402566271

To convert, you can give date a specific date using the -d flag. However, to get a Unix date, this needs to be a full date. You can't convert 3:00PM to Unix time since Unix time refers to an entire date (year,month,day,time). So, for example, to get the Unix date for the 12th of September 1987, you would do:

$ date -d "3 PM 12 September 1987" +%s
558450000

And to convert that back to a "normal" date:

$ date -d "@558450000" 
Sat Sep 12 15:00:00 CEST 1987

I didn't find a simple solution to my question, so I've written a small Bash script to solve it. You need to download the leap seconds file given in the link below and put it with the script or change the path to it. I didn't write utc2unix.sh yet, but it's very easy to adapt. Do not hesitate to comment/give suggestions...

unix2utc.sh:

#!/bin/bash

# Convert a Unix timestamp to the real number of seconds
# elapsed since the epoch.

# Note: this script only manage additional leap seconds

# Download leap-seconds.list from
# https://github.com/eggert/tz/blob/master/leap-seconds.list

# Get current timestamp if nothing is given as first param
if [ -z $1 ]; then
    posix_time=$(date --utc +%s)
else
    posix_time=$1
fi

# Get the time at which leap seconds were added
seconds_list=$(grep -v "^#" leap-seconds.list | cut -f 1 -d ' ')

# Find the last leap second (see the content of leap-seconds.list)
# 2208988800 seconds between 01-01-1900 and 01-01-1970:
leap_seconds=$(echo $seconds_list | \
               awk -v posix_time="$posix_time" \
               '{for (i=NF;i>0;i--)
                   if (($i-2208988800) < posix_time) {
                    print i-1; exit
                    }
                } END {if (($(i+1)-2208988800) == posix_time) 
                    print "Warning: POSIX time ambiguity:",
                            posix_time,
                          "matches 2 values in UTC time!",
                          "The smallest value is given." | "cat 1>&2"
                }')
# echo $leap_seconds

# Add the leap seconds to the timestamp
seconds_since_epoch=$(($posix_time + $leap_seconds))

echo $seconds_since_epoch

Just some tests:

• date --utc +%s && ./unix2utc.sh -> today and at least until June 2015 the difference is 25 sec.
• ./unix2utc.sh 78796799 -> 78796799
• ./unix2utc.sh 78796801 -> 78796802
• ./unix2utc.sh 78796800 -> 78796800 + on stderr: Warning: POSIX time ambiguity: 78796800 matches 2 consecutive values in UTC time! Only the smallest value is given.