When/What is the expiration date?
I've always said "when" but I'm not really sure which one is correct. Are both of them correct?
Which one is correct and why?
"When is the expiration date?" and "What is the expiration date?" are both correct.
"What" is a lot broader and can be applied to more things than "when".
The first example using "when" is asking the question with an emphasis on time. While the second example using "what" is emphasizing the noun "expiration date".
Here is an example of how the uses of those two sentences could differ:
When is the expiration date?
You could answer this with: "In two weeks", "Tomorrow", "April 7th".
What is the expiration date?
This can ONLY technically be answered with a date such as: "April 7th".