What is the rolling budget and zero based budgeting

The rolling budget is a budget which is cautiously updated by subtracting the period that has already passed from it and adding a new period to it. For example, a three-months-rolling budget for Jan-March will be updated at the end of Jan by subtracting January's budget from it and adding April's budget to it. This way, you will always have a three-month budget for the next three months. 

Zero based budgeting is the process of starting the budgeting process from scratch and having to justify every single cost again, as opposed to updating and modifying our last budget. This budget is more accurate and it forces consideration of the costs and whether they're justifiable or not, but it takes a considerable amount of time and human resource to be prepared.