Set Field ( Creation Date; "<" & Get ( CurrentDate ) )
No need to use Insert Calculated Result
Beverly's is the simplest but if you wanted more flexability you could do:
Enter Find Mode 
Insert Calculated Result [Select; Creation Date; Get ( CurrentDate ) - 31 & "..." & Get ( Current Date )-1]
You could create different scripts for different time periods if you needed them e.g last week, month, 3 months etc. and there are many other ways of doing it using globals to make it even more flexible.
This isn't entirely on-topic, but the various posts here with <, > and ... reminded me to share.
A client actually pointed out to me only a couple of years ago that searching for '1...' is the same as searching for '>=1', and that '...10' is the same as '<=10'.
Not really a big deal most of the time, but for all the times a script is going to have a begin date and/or an end date, it saves from having to do a case for begin only, end only, or both. just $$begin & "..." & $$end, and no worries if one of them is blank.
Just one more thing about FM dates, and doing finds, that I spent way too long working way too hard for.
+1 Chris. I learned something from you today!