grep substring between quotes

With GNU grep, you can do:

$ echo 'title="2010-09-11 11:22:45Z"' | grep -oP 'title="\K[^"]+'
2010-09-11 11:22:45Z

grep -oP '[0-9-]{10} [0-9:]{8}' filename

If input would be in this format only then below command will easily solve your problem

echo "title=\"2010-09-11 11:22:45Z\"| cut -d '"' -f2