Calendar times aren't right

For the second time this week someone has signed up for a one-on-one conversation with me after 9pm. This is very odd as I don’t offer conversations after 8:30pm. Is this a bug?

Hi Helen, I can’t make that happen on the site. It obviously shouldn’t be able to happen. Is it the same person who is able to do this? If so, please let me know what their time zone is. That could be the cause.