VBA to define alias for workbook location to be used in formula

Hi folks,

The hospital I work for provides an excel workbook for each employee in which they have to keep track of their working hours, sick-leave, holidays etc (called a jaarkaart in Dutch).
I'm trying to make a separate excel workbook in which an automatically updated overview will be made of the data each of the people in our department fills in in his/her jaarkaart.
Each day is represented in the jaarkaart by a block of 4 cells, the top two of which are used to fill in the working hours (am and pm) and the bottom two are used for codes to indicate illness, days off etc (see attached example).

Currently, my overview spreadsheet is filled with formulas such as this:

=IF(OR('I:\secretary\holidays\holidays 2016\[jaarkaart-2016 Thirsa.xlsx]Sheet1'!C$12="";'I:\secretary\holidays\holidays 2016\[jaarkaart-2016 Thirsa.xlsx]Sheet1'!D$12="");SUM('I:\secretary\holidays\holidays 2016\[jaarkaart-2016 Thirsa.xlsx]Sheet1'!C$12:D$12)/2;'I:\secretary\holidays\holidays 2016\[jaarkaart-2016 Thirsa.xlsx]Sheet1'!C$12)

Four of these per day for each employee.....
So you can probably imagine that even for our small group of 12 people, when I try, for example, to update 2016 to 2017 by find-replace all....excel crashes because that's too much work.

Therefore, I was wondering if it is possible to define aliasses for the locations of the jaarkaarten in VBA and use those aliasses in the formulas, so that the formulas will end up looking something like this:

=IF(OR(Thirsa!C$12="";Thirsa!D$12="");SUM(Thirsa!C$12:D$12)/2;Thirsa!C$12)

I tried making a private sub which basically contained the following for each employee:

Dim Thirsa As Sting
Thirsa = I:\secretary\holidays\holidays 2016\[jaarkaart-2016 Thirsa.xlsx]Sheet1

But that didn't work.

Hope you can help me out.
Thanks in advance.
Thirsa