Hi bottleknows,

If you're connecting your bank account to download your transactions, by default, it downloads your transactions in the last 90 days. However, you can just select the transactions you want to add in the last 30 days, and then exclude the other transactions.

If you're bank is not connected, you can down your transactions in 30 days and upload them to QuickBooks. 

Feel free to comment below if you need more help.