Linux – How to Set Time with UNIX Timestamp Using BusyBox Date Command

You ask for "known workarounds." Here is a simple one:

$ date -d "$(echo 20140103T1422 | sed 's/T/ /')"
Fri Jan  3 14:22:00 PST 2014

This uses sed to replace "T" with a space. The result is a format that date understands.

If we add seconds onto the ISO8601 date, then date requires more changes:

$ date -d "$(echo 20140103T142211 | sed -r 's/(.*)T(..)(..)(..)/\1 \2:\3:\4/')"
Fri Jan  3 14:22:11 PST 2014

In the above, sed replaces the "T" with a space and also separates HHMMSS into HH:MM:SS.

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.