How do I get difference between two dates that are in yyyy-mm-dd format

A few days ago one of my friends asked me - How to get the difference in terms of number of days from two dates stored in yyyy-mm-dd format. I was in a hurry and told him how to go about, but couldn't show him the exact steps.

Yesterday I posed this questions to the interns in our company, thinking they would benefit from solving it. I told them to read the date values from shell environment variables, and use the programming language /tools of their choice to prepare the solution.

One guy wrote a neat C code to get the answer. He had to maintain a data structure to store how many days are present in all the months of the year, and also a function to find if the given year is a leap year or not. That resulted in the code growing to about 100 lines.

I asked them if they could think of some other approach, and if they were aware of how time is maintained in UNIX systems.

A perl script that uses package Time::Local makes the task easier to accomplish.

#!/bin/perl
use strict;
use warnings;
use Time::Local;

my $date1 = $ENV{'date1'};
my $date2 = $ENV{'date2'};

my @date1_breakup = split(/-/, $date1);
my @date2_breakup = split(/-/, $date2);

my $date1_unix = timelocal(0, 0, 0, $date1_breakup[2], $date1_breakup[1], $date1_breakup[0]);
my $date2_unix = timelocal(0, 0, 0, $date2_breakup[2], $date2_breakup[1], $date2_breakup[0]);

my $diffSeconds = $date2_unix - $date1_unix;
my $diffDays = $diffSeconds / (60 * 60 * 24);
print "difference in days : $diffDays\n";


A plain simple bourne shell script can also do the job for us, if the date command supports -d option.

#/bin/sh
date1_unix=`date -d $date1 +%s`
date2_unix=`date -d $date2 +%s`
diff=`expr $date2_unix - $date1_unix`
diff_days=`expr $diff / 86400` # Number of seconds in a day are 86400
echo $diff_days

yogeshs@yogesh-laptop:~/temp$ export date1=2010-04-22
yogeshs@yogesh-laptop:~/temp$ export date2=2010-05-23
yogeshs@yogesh-laptop:~/temp$ ./date_diff.sh
31
yogeshs@yogesh-laptop:~/temp$

Here's what happened inside the shell script.
yogeshs@yogesh-laptop:~/temp$ sh -x date_diff.sh
+ date -d 2010-04-22 +%s
+ date1_unix=1271874600
+ date -d 2010-05-23 +%s
+ date2_unix=1274553000
+ expr 1274553000 - 1271874600
+ diff=2678400
+ expr 2678400 / 86400
+ diff_days=31
+ echo 31
31
yogeshs@yogesh-laptop:~/temp$


You still want to see how this is done using C? Do let me know and I'll share that piece of code with you.