Formula revealing # of days b/t dates that will read future data (inserted by macros)

I have a Master Data sheet that stores all data from my bakery. In the same workbook, I have a Copy Data Here Page where I insert the excel file downloaded from our Point of Sale system and after running the macro, it converts it all into the right format and drops it in the Master Data Page. I also have a Sum Page in this workbook that I want to read a variety of summarized data (by manually inserting a start date and end date in cells A3 (Start Date) & B3 (End Date)). The first thing that I would like to summarize is the # of days between the dates entered on Sum Page C3. For instance, if start date is January 10, 2019 & end date is January 11, 2019, the answer would be 2 days. All the dates are in Column C of the Master Data Worksheet. There are two dilemmas: first, there is a date assigned to every item sold which means that if I sold 10 pies on January 10, 2019, Column C will have 10 dates of January 10,2019. The second problem, is that I do not want to have update the formula every time new data is inserted into the Master Data page from the macro. I would like a formula which handles all these problems. Help would be appreciated. Thanks!

There are many ways to approach your problem but I don't see you posted a specific formula question for us to help.

Try adding a Pivot table to your data and explore its functions. Every time you have new data added, all you have to do is refresh your Pivot table.

The formula I need will need to calculate the # of unique dates between 2 dates on a different sheet and be able to read new data as inserted.