Sorry Kimberly, all you said is absolutely true, except this: " Create a formula =A1-A2. the answer is how many days til your next birthday."
The problem is that A1-A2 is a negative number, and the result is the age in days , and not how many days til your next birthday.

As for kjxavier problem, my guess is that the cell in which you have directly entered date as 01/05/2008 is formatted as text, and not as date, this is the reason excel says the dates are different.
You can solve the problem using in your comparrison formula to convert text to date:
=DATE(RIGHT(L16;4);MID(L16;4;2); LEFT(L16;2)) (-just replace L16 with referrence to other sheet) , and now, when you compare the date in this sheet with the result of the formula, will say they are the same...
Hope this solves your problem...