Timekit works by scanning a calendar to determine what is free and what is busy. It then carves the free times up into bookable slots based on the rules you've set.
A word on free/busy
Both Google and Microsoft (and most calendar apps) allow you to create events in your calendar and mark them as "free" or "busy." From our experience, we see it as a result of the fact that many people also use their calendar as a "to do list" and since most calendars have a simple built-in availability checker to avoid conflicts when setting up meetings with colleagues, they allow you to set certain events as "free."
All-day events are free by default:
We are not exactly sure when or who decided it, but all-day events, in both Google and Microsoft, are set as "free." That means if you are using an all-day event to block yourself for vacation, out-of-office or anything similar that you want to exclude from your available time, remember to set it as busy