I am trying to create a Shopify report to show how many orders have shipped per day for a given date range. Currently there is no way to indicate what date to use so the report is wrong. My assumption is the dates are aggregated on the ORDER date, not the FULFILLMENT date. (even though I'm filtering based on FULFILLMENT STATUS)
The reporting function built into Shopify regarding orders will typically be based on order date. If you are looking for a way to arrange this data dependant on fulfillment date, I would suggest exporting a CSV File of the orders.
You can use the spreadsheet editor to highlight the number of cells that hold the same date, and the number of cells reported there will tell you how many orders went out that day. You will also have acces to the rest of the raw data here, like order number and cost of order.
I hope that helps! Please let me know if you have any other questions about this :)