Shell Programming and Scripting

#!/bin/bash
sed -e "s/-//g"  file > tmp
t=`date "+%Y%m%d"`
awk -F',' '{s=$(date -d"$4" +%s); j=$(date -d"$t" +%s); k=$($($($j-$s)/3600)/24); print $1","$2","$3","k}' input > output

awk -F, -v OFS="," '{ split(/-/, A, $3); T=systime()-mktime(sprintf("%04d %02d %02d 00 00 00", A[1], A[2], A[3]));
        T/=(60*60*24);
        $3=T;        
 } 1' inputfile

  gawk -F, -v OFS=, '{ split(/-/, A, $4); T=systime()-mktime(sprintf("%04d %02d %02d 00 00 00", A[1], A[2], A[3]));
        T/=(60*60*24);
        $4=T;        
 } 1' inputfile

$ cat inputfile
John,Miami,,2013-10-03

$ date
Fri Oct  4 23:17:36 AST 2013

$ bash -x scr.sh
+ gawk -F, -v OFS=, '{ split(/-/, A, $4); T=systime()-mktime(sprintf("%04d %02d %02d 00 00 00", A[1], A[2], A[3]));
        T/=(60*60*24);
        $4=T;        
 } 1' inputfile
John,Miami,,735177

gawk -F, -v OFS=, '{ split($4, A, "-"); T=mktime(sprintf("%04d %02d %02d 00 00 00", A[1], A[2], A[3]));
        $4=int((systime()-T)/(60*60*24));
 } 1' inputfile