If I create a Scheduled View with a start date in the past and data forwarding enabled, how will the data be forwarded?

Data forwarding will forward current data going forward and historical data in reverse order.

For example, let's say that you set up the start date to be 30 days in the past. 

So as per above the newer historical data (e.g. 1 day old) will be forwarded before older historical data (e.g. 5 days old).