Rebbe Malachi: Re: function definition for only one file, not for other simultaneously opened files

I am struggling to find a way to deal with the following problem.

Suppose, I open 3 markdown files at once in separate tabs. Now I want some particular function for one of those files only. Like, I want to add time and date in front of each new line for that file. So I define a function to do so. But, that function is affecting other files too which I don't want.

Is there some way to achieve that?