When writing code for someone else, on a money per hour basis, you have to make choices that are in the clients best interest. Usually best interest is the cheapest solution that works. One example of this choice is perfection versus practicality.
When writing code for a single task, if the code meets the requirements for the task, it "works". If you have to write a function to parse a number, which launches nuclear missiles if given a negative number, and you don't give it negative numbers, then that's fine. If you spend additional time making it robust to negative numbers, you are wasting someone else's money.
If you are writing beautiful code, or a library (where people can poke the interface in unexpected ways) then handling side conditions is very important. The fewer obscure side conditions, the fewer surprises to the users. Fewer surprises means less need for documentation and fewer help emails.
As projects become larger, typically an increasing proportion of the code becomes a library which the remaining section of the program makes use of. Once this happens, it is worth going back and reconsidering and refining the interfaces to functions. Judging when a program goes from "quick hack" to "large system" is hard. The best time to go back and make this refinement is when the code is still small, but you know it will be growing shortly. Alas, this is exactly when deadlines are likely to make refactoring impossible.
So back to the title of the post, the year 3000 problem. I was writing code to search through a list for items with a particular year. The list goes back to 2005 only. I wrote a function which given a blank string returns the latest ten items, and given a year, it returns all the items which have that year. I implemented the year check using substring searching (indexOf/InStr/strstr - depending on your language). There are reasons for using substring searching rather than simple equality, but they are not relevant here. You can write this code as:
var items = complicated_code_here();
for (var i in items)
if ((year == "" && i < 10) ||
(items[i].year.indexOf(year) != -1))
Simple enough. Now its very easy to add a facility to call the search function with the appropriate year and display the results. Now imagine that you get a change request, asking for a button to display all results. What would you do? Remember that you are looking for the solution that meets the clients needs with the least amount of work.
The answer I came up with, simply call search("2");. The code works, and will continue to work for 992 years. In the year 3000 there will no longer be a 2 in year field, and it will stop working. By then, I'm expecting to be dead, and I hope to outlive VBScript by decades, so this isn't something that will ever be a problem.
When learning to code it is important to think about the corner cases, and to deal with them appropriately - always aiming for perfection. Once you have a reasonable idea what the "perfect" solution looks like, then you need to start making concious decisions about whether perfection is the right choice. Usually it is, but sometimes you need to make compromises for the benefit of the person paying.