General structure for often re-used functions, etc.

Hi :) General newbie question here.
I'm just wondering how the general population of more experienced developers goes about handling scenarios like this:
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I do my own error checking for dates.  I do my own loops and character checks to make sure it is strictly in the form of ####/##/##.  I call this function "DateErrorCheck()" and it returns a boolean value.  It's only a few lines, but I find I re-use this code on every page which calls for entry of a date.  So I have about 5 pages in my website which use this exact same code, just copy/pasted into each .aspx page's code behind.  My question is, how can I centralize this so that every page calls the same code, and if I ever need to change it, I only have to change it in one place?
For actual applications, Windows Forms, etc., I understand (though have no experience with) that you can use .dll's, reference that, and then you're done.  And ...