Date and time calculations to track time using command line in Unix/Linux

echo "$(date) : Start" >>worktime.txt

echo "$(date) : End" >>worktime.txt

kent@rat:~/work$ cat worktime.txt
Fri Jan  2 19:17:13 CET 2009 : Start
Fri Jan  2 19:47:18 CET 2009 : End
Fri Jan  2 21:41:07 CET 2009 : Start
Fri Jan  2 22:39:24 CET 2009 : End

#!/bin/bash

total=0
while read LINE; do
  d1=$(echo $LINE |cut -c1-28);
  t1=$(date -d "$d1" +%s);
  read LINE
  d2=$(echo $LINE |cut -c1-28);
  t2=$(date -d "$d2" +%s);
  total=$(($total+$t2-$t1));
done
echo $(($total/(60*60))) hours

echo $(date +%s) : Start

#!/bin/bash

total=0
while read LINE; do
  t1=$(echo $LINE |cut -d' ' -f1)
  read LINE
  t2=$(echo $LINE |cut -d' ' -f1)
  total=$(($total+$t2-$t1))
done
echo "$(echo "scale=2; $total/(60*60)" |bc -l)" hours

BEGIN {
    total_secs = 0
    FS = OFS = ":"
}

function snip_it() {
    $NF = ""; $0 = substr($0, 1, length($0)-1);
}

!(NR % 2) {
    snip_it();
    ("date -d "" $0 "" +%s") | getline start_secs
}

NR % 2 {
    snip_it();
    ("date -d "" $0 "" +%s") | getline end_secs
    total_secs += (end_secs - start_secs)
}

END {
    print "Total work time of " total_secs " seconds"
}