Hi jendimos,
The dates are based off of the first lodgement recorded in the GST Centre and move forward chronologically from there. When you first click 'Get Started' to prepare the first BAS, the GST Centre will initially prompt you to enter the dates of the period you'd like to lodge. If this process was initiated but the BAS was not lodged, you should still be able to exit the setup and begin again with the dates of your choice. If however a period was lodged or marked as lodged (check in either the To do or History tab of the GST section) this will need to be unlodged so you can start over. An accountant user on the file will be able to do so, or you can reach out to our Support Team directly on 1800 046 038 to unlodge that period for you.
Feel free to reply if you have further questions as I'm here to help!
-Kass
