Someone

#!/bin/bash
IFS="
"
for i in $( cat txt.txt ); do
    CONTAINSDATE=`echo $i | grep "pubdate"`
    if [ "$CONTAINSDATE" != "" ]
    then
	INPUTDATE=`echo $i | cut -d ">" -f 5 | cut -d "<" -f 1`
	FIXEDINPUTDATE=0$INPUTDATE
	OUTPUTDATE=`date -R -d $FIXEDINPUTDATE`
	echo $i | sed -e s/$INPUTDATE/$OUTPUTDATE/
    else
	echo $i
    fi

done