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

while read DATE TIME REST
do
	case "$DATE $TIME" in
	200[3-4]-[0-9][0-9]-[0-9][0-9] [0-9][0-9]:[0-9][0-9]:[0-9][0-9])
		# format $DATE $TIME into epoch time
		BAD_DATE=`date -j -f "%Y-%m-%d %H:%M:%S" "${DATE} ${TIME}" "+%s"`
		# add 246471660 seconds to $BAD_DATE
		ADJ_DATE=`expr ${BAD_DATE} + 246471660`
		# re-format $ADJ_DATE back to original format of $DATE $TIME
		FIXED_DATE=`date -j -f "%s" "${ADJ_DATE}" "+%Y-%m-%d %H:%M:%S"`
		# replace $DATE $TIME with $FIXED_DATE
		sed "s/${DATE}/${FIXED_DATE}/"
		;;
	*)	# Do nothing
		;;
	esac

	echo "$DATE $TIME $REST"
done < test.txt > test2.txt




$ ./date_configurator.sh 
./date_configurator.sh: line 6: syntax error near unexpected token `[0-9][0-9]:[0-9][0-9]:[0-9][0-9]'
./date_configurator.sh: line 6: `	200[3-4]-[0-9][0-9]-[0-9][0-9] [0-9][0-9]:[0-9][0-9]:[0-9][0-9])'

#!/bin/bash

echo "Enter file name that contains corrupt dates and press [ENTER]"
read file_name
output=`echo "$file_name"_edited`

while read DATE TIME REST
do
if [[ "$DATE" =~ 200[3-4]-[0-9][0-9]-[0-9][0-9] ]]
then
  BAD_DATE=`date -j -f "%Y-%m-%d %H:%M:%S" "${DATE} ${TIME}" "+%s"`
  ADJUSTED_DATE=`expr ${BAD_DATE} + 246471660`
  FIXED_DATE=`date -j -f "%s" "${ADJUSTED_DATE}" "+%Y-%m-%d %H:%M:%S"`
  echo "$FIXED_DATE $REST" >> $output
else
  echo "$DATE $TIME $REST" >> $output
fi
done < $file_name