Records Select Expert - Maximum Date

I have a report where I use a parameter to select a reporting period (which is a field in our database). Example: Jan 2016 (a string field)

I want to automate this report, so it will automatically use the "latest reporting period" without entering any parameters.

There is another field I can use called "First Date of Reporting Period". (datetime field)

I tried a maximum formula using the "First Date of Reporting Period" in the Select Expert, but this does not seem to be allowed.

Any suggestions?

Thanks.