You can set the 12 month budget start in that first budget month column on the Categories sheet to something other than Jan 2019. If you were to set it to 9/1/2019 your 12 month budget would start Sept 2019 and end Aug 2020.
You can also extend the budget by adding additional columns to the right of the last month in the Categories sheet. For a 24 months, add an additional 12 columns to the right and then use the quick fill square to drag the formula from last last month header to the right for those other columns.
Then your yearly and monthly budget view tabs will be able to pull data in from those future months.
Video demo of how to change start month here:
Let me know if you get stuck.