DRY principle - Don't Repeat Yourself. Say you write a few lines of code to get yesterday's date, and need use it over and over in a large application---a simple example:
You could copy and paste that code into other places in your code blocks where you need it, or into separate source files, even. Or have a single copy that lives in just one function in one piece of code that all of your othere code references, passing some kind of string to fill in with "yesterday". Maybe like this:
strftime(yesterday, 32, "%B %d", localtime(<));
The DRY principle refers to the second option. It means you have to go to one place to validate or check your yesterday code, one place to make changes. Instead of going on a hunting expedition for 10 places (or was it 11?) sprinkled everywhere. And making 10 or 11 changes.
char *yester(char *dest)
strftime(dest, 32, "%B %d", localtime(<));