Please provide more detailed description of the task and some example test cases as well.

Thanks :-)

See here: http://jsfiddle.net/J3Spt/
Maybe it's not much shorter, but at least it's algoritmic :-)

If year is e.g. 2000 (non-leap), this solution will return incorrect result - true, I think :(

@ca6age:

var handled = ending.replace(/[.+]/g, '\$&');
return new RegExp(handled + "$", "i").test(str);

:-) You could replace your function num with:
num_code_of_x = x.toLowerCase().charCodeAt(0);

Same here :(

Please provide Test object (or info that it's present) for user's own test cases

Will not work for test cases where number < 0, I think.

What is purpose of "~~" in this case?

Wow, really elegant solution :)