'Expiration date' question

Hello all,
we have a little discussion about the expiration date when sharing files. As an example: I share files and set an expiration date 18.12.2017. Do this mean that the files are not available starting 18.12.2017 0:01 o’clock? Or do this mean that the files will be not available at 18.12.2017 23:59 o’clock?

Thank you all!