I would like to use tar comamnd to do the below thing.

tar files which are created ( or last modified date ) today and its file extension are .txt .

can advise what can i do ?

Thanks.

Use the find command with '-exec tar -uvf <filename>'

Look through the tar info manual. (info tar). Your best bet is probably to use the find command to locate the files and create a file of filenames to backup. Then set the FILELIST variable to the name of this file. If the number of files to backup isn't very large you could set the BACKUP_FILES variable with the names of the files to backup. If some of the filenames may contain whitespace, use "-print0" with the `find' command and with `tar' use the --null option.

example

find basedir/ -daystart -type f -name "*.txt" -ctime -1 -print0 >/tmp/filelist
tar -C basedir/ -cvf backup-$DATE.tar --null -T /tmp/filelist