US income has a base of 6 income starting in 36’. Only thing that could change that income is the peace time income bonuses.
Lets pretend Japan declares war on China that would then make US income 11 every turn unless other peacetime bonuses are triggered.
Then in Jan 39’ US gets to start rolling a D12 and rolls a 8. Making the US income 19.
No other peacetime triggers occur so then July 39’ comes along with a now base income of 19 he gets to roll a D12 again but this time rolls a 5. Making his income 24 now.