Back projections question

When you start a game, it is back projected so that you do not start with the exact values in the mission file, but they are worked out based on the effect of your policies over the last few months/years. Thus although you may set debt to 0, if your income and expenditure do not balance, it may not start that way.

My question is: how long is the back-projection done for? Is there a simple answer like “two turns”?