Could I, first of all, encourage you to press the Thanks button for rdrtx1's very useful post.
Getting to your issue, what have you got to work it in to? There must be something else that you are running that you want to use it with. What have you tried so far?
It would be good to know your OS version and your preferred tools (e.g. ksh, bash, perl etc.) so we can help you fit it in a way that you can support in future.
#run interactively
a="2017-07-12_gmr_tag_log_20170711.csv"
echo $a
#remove the first 11 characters
b=${a:11}
echo $b
#set l to the length of string b
l=${#b}
echo $l
#set s to the middle length of string b
let s=$l-13
echo $s
#set c to the middle part of the string
c=${b:0:$s}
echo $c
#concatenate string c and ".csv"
d=$c.csv
echo $d