Get Data From Cells In External File - File Name Changes Daily

I have a file that gives some statistical data to my co-workers every 15 minutes. A common question I get is "How does that compare to last week?" Then I have to open the file from 7 days ago, find the data from the same time interval, and subtract it from this week's number in my head. I'd like to have excel do this for me.

I know how to get data from an external file. The problem is, these files are named with a date on the end of the file name. So tomorrow, the static formula won't work anymore (or rather, it will give data for a file from 8 days ago, instead of 7). I'd like to excel to use today's date, find the file from 7 days ago, and get the data from that file to compare to the current file.

Here's what I've done so far:

Code:
ThisDate = Range("C1").Value 'the cell with todays date in it
ThisDateName = Format$(ThisDate, "yyyy-mm-dd") 'now formatted the way I need it
ThisDate7 = Range("G1").Value 'the cell with the date 7 days ago
ThisDateName7 = Format$(ThisDate, "yyyy-mm-dd") 'formatted correctly
ThisDate14 = Range("G2").Value 'the cell with the date 14 days ago
ThisDateName14 = Format$(ThisDate, "yyyy-mm-dd") 'you know, in case of a holiday 7 days ago

I have no clue what to do next. I want Excel to: Use "ThisDateName7" to find the file with the name "pph_tracker_[ThisDateName7].xlsm"Get data from a cell in that file (say, C15)Subtract it from the data in the same cell (C15) in today's file (this week - last week)Give me the result in today's file (say in cell C20)Do that again for cells D15, E15, and so on (result in D20, E20, etc)

I assume I can figure out the rest from there. Can I use the variable names in an actual formula in cell C20? Something like:

=C15 - '[pph_tracker_{ThisDateName7}.xlsm]Sheet1'!C15
or even
=C15 - '[pph_tracker_{The Date in Cell $G$1}.xlsm]Sheet1'!C15
which would require no macros at all!