#!/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  

:( thanks for trying

date: illegal option -- R
usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ...
            [-f fmt date | [[[[[cc]yy]mm]dd]HH]MM[.ss]] [+format]