John:
You're correct that the setting Andrew suggested isn't an option in the GL (it is in other modules). You could address this as a short-term solution by doing the following:

* On the Selected fields tab, insert "Filler" where the Period exists in the source file. Click on the "Filler" button and move it to the correct sequence
* On the "Default" tab, enter the correct Period number.

However this would not be my first suggestion. It has 2 problems: i) it only works if all transactions in the batch have the same Period and ii) you have to remember to change it every month.

I would go back to the 3rd party developer and ask under which conditions the Period would be output as a "0" (i.e. new fiscal year not set up yet?). It's not enough to say that the other appplication hasn't changed; neither has the Adagio GL!

Do you still have any of the past export files? Check the Period in those to prove that it wasn't 0 in past batches.