Global constant across whole module

I have a stack of custom formulas that have grown over the years. A lot of these are year specific, so a call passes a year argument to one of the functions to pick up the correct details. In order to deal with years that aren't covered I've always started with:

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clearly depending on the last year.
The problem is when I update these I need to change that line over and over - and then sometimes forget. What I would like to do is set a global variable so I could change that to something like:

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.

I tried in the module setting Public final_year as integer and then final_year=2017 but that ain't right. How do I do it? I am sure it is simple form someone that knows more about this than me.

Thanks in advance.

Many thanks - worked a charm. Although threw up something I was not at all expecting elsewhere. Typical huh?