you can maintain the "fiscal month","from date" and "To date" mapping in a lookup.
refer the lookup when user selects a particular fiscal month from drop dow.
for eg your look up has columns as
Month From To
Sep 8/25/2018 9/25/2018
when you get the value of From and to date, use strptime to convert into splunk time format ,
eval earliest=strptime(From,"%m/%d/%Y")
eval latest= strptime(To,"%m/%d/%Y")
Once you get earliest and latest time value, assign it to a token and refer the token within your search query and tags
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