It looks like you are looking for a single formula solution but they could be hard to understand and maintain, specially if you aren't familiar on how the Google Sheets related features works.
Dates. Google Sheets handles dates as serialized numbers, where a day is one unit.
Because of the above feature we could create a list of dates by ...
This formula sums all the sums in column C by date:
=sort(transpose(QUERY(inservice!A3:C200, "select SUM(C) where A is not null pivot A", 0)), 1, FALSE)
You might want to check the QUERY function, which is what solves your problem. I used TRANSPOSE and SORT in order to keep your presentation style.
Google Sheets built-in functions are not support on Google Apps Script.
Below is a function that returns the same result as TODAY built-in function:
* Returns a date object with the time set to 12:00:00 AM which is the same
* as Google Sheets TODA() ...