Hi Niko,
As mentioned in the first part of that posting, the formula syntax is not specific to a data file, and {entry date} is not the name of a spcific field. This is just an example of how to isolate transactions that exist in a prior month.
You have 2 choices for which file to select. If you want to point at AP, use the table 'AP GL Disbursements', which contains all the GL distributions of transactions posted by Adagio. Note that {Entry date} appears here.
Your other choice is to point at the GL Posted Transactions table. The date is called {Trx Date} and you would need to include a filter against the field {Ledger} in order to select only those records that came from AP (or AR or OE etc).
Which choice you make depends on what information you want - it's up to you.