use or Private constant to define a fixe file name?

To all

playing around with a code which output a string to a text file using the StreamWriter(). A the moment the code loops around a series of input, create some results, extract specific data, format the data and write the formatted data to a text file. Work like a charm (-ish)
In effect each loop create a text file which has a unique name using

Dim createdTime As DateTime = DateTime.Now
theFileName = LCase("" & theRespSimName & "_" & theEventName & "_" & Format(createdTime, "ddmmyy_hhmmss") & ".log")

I am thinking/playing around with the idea of having an (user) option to have only 1 output file written at the end. I have therefore modified the code so that the 'appendmode' is set based on the user's choice (if 1 output file wanted then append mode=True, else false)

The catch is that I do not want the file name to be something like "mycombineddata.txt" otherwise everytime the macro is run (and the file exists in the folder) it will append the data.

I am therefore thinking about a constant for the file name which is set at the 1st pass/loop, something like,

Private Const theFileNameFixed As String = LCase("combineddata_" & Format(createdTime, "ddmmyy_hhmmss") & ".log")

My thinking is that every time the code is run a unique combined file is created

The code does not like this syntax. What is the correct syntax to use (private!) constant?

Thanks
Regards
JXB