$ cat test.txt 
# the actual log files hold lines that DO NOT include dates
# as i said above the lines that DO NOT include dates also must be 
# maintained across to test2.txt 
2004-08-14 18:49:43 [2e0-00000fe0] LOG asconfigurator is exiting normally
2003-08-14 18:49:43 [2e0-00000fe0] LOG asconfigurator is exiting normally

$ cat date_configurator.sh 
#!/bin/bash -x

# get $date
grep -o "200[3-4]-[0-9][0-9]-[0-9][0-9] [0-9][0-9]:[0-9][0-9]:[0-9][0-9]" test.txt | while read date
do
      # format $date into epoch time
      bad_date=`date -j -f "%Y-%m-%d %H:%M:%S" "${date}" "+%s"`
      # add 246471660 seconds to $bad_date
      adj_date=`expr ${bad_date} + 246471660`
      # re-format $adj_date back to original format of $date
      fixed_date=`date -j -f "%s" "${adj_date}" "+%Y-%m-%d %H:%M:%S"`
      # replace $date with $fixed_date
      sed "s/${date}/${fixed_date}/" <test.txt >>test2.txt
done

$ ./date_configurator.sh 
+ grep -o '200[3-4]-[0-9][0-9]-[0-9][0-9] [0-9][0-9]:[0-9][0-9]:[0-9][0-9]' test.txt
+ read date
++ date -j -f '%Y-%m-%d %H:%M:%S' '2004-08-14 18:49:43' +%s
+ bad_date=1092523783
++ expr 1092523783 + 246471660
+ adj_date=1338995443
++ date -j -f %s 1338995443 '+%Y-%m-%d %H:%M:%S'
+ fixed_date='2012-06-06 11:10:43'
+ sed 's/2004-08-14 18:49:43/2012-06-06 11:10:43/'
+ read date
++ date -j -f '%Y-%m-%d %H:%M:%S' '2003-08-14 18:49:43' +%s
+ bad_date=1060901383
++ expr 1060901383 + 246471660
+ adj_date=1307373043
++ date -j -f %s 1307373043 '+%Y-%m-%d %H:%M:%S'
+ fixed_date='2011-06-06 11:10:43'
+ sed 's/2003-08-14 18:49:43/2011-06-06 11:10:43/'
+ read date

$ cat test2.txt 
2012-06-06 11:10:43 [2e0-00000fe0] LOG asconfigurator is exiting normally
2003-08-14 18:49:43 [2e0-00000fe0] LOG asconfigurator is exiting normally
2004-08-14 18:49:43 [2e0-00000fe0] LOG asconfigurator is exiting normally
2011-06-06 11:10:43 [2e0-00000fe0] LOG asconfigurator is exiting normally