[donkeyd]

Microsoft, who provides our mail and calendar service, has a tool for this available in Office 365. So no need to connect an outside service. Otherwise, self-hosting is an option. Or you make the decision that the risk of using a third party is alright in your business and do that.

[jon-wood]

It's also built into Google Calendar, create an event across some time, then change its type to Appointment Schedule.

[SturgeonsLaw]

What's the tool?