Simcity graphs are all represented in months, but there doesn't seem to be ~30 days in a month. How many days are in a month?
A month is the same as a day. Therefore there are 12 days (months) in a year.
There are 24 hours in a budget month. You can confirm this by looking at the budget window.
"Profit/Loss" * 24 + "Monthly Transactions" = Monthly Profit
Also, you can see the current month and year by hovering over the clock in the lower left corner.