Returns: exactly what is the date you can return and ship?

I understand you have 14 calendar days to return an item.

I received my Apple Watch on the 16th and the receipt says I can return it by the 30th.

My exchange watch is arriving on the 30th and I am unsure which one I will want to keep.

My question is, what exact date could I return the first one?

Must the return process be performed on the 29th, or the 30th?

I would like to see the exchange one before committing to return the first watch.

Also, how long do I have to ship it back to Apple?

Thank you for any help.