The Transactions sheet in all Tiller spreadsheets have the "Month" column as a default that is populated by the automated data feeds. Usually you will need to scroll to the right in your Transactions sheet to see it.
ℹ️ Below steps are for Google Sheets only.
You might need to manually add month data if you're preparing historical data that you want to manually add to your Transactions sheet, or inputting transactions for accounts Tiller can't automate, you can use the below arrayformula to have it automatically fill in the month data for the data set in a "Month Array" column.
ℹ️ This assumes the "Date" is in column B (the default in the Foundation template). Change the formula to match the location of the date data in your prep sheet.
Once you have generated the month data needed, copy the data and then right click to Paste Special > Values Only into the automated Month column in the Transactions sheet.
Optionally, remove the Month Array column after you're done using it.
For best performance results use Tiller's automated Month column rather than using an arrayformula to populate the month data.