Outlook could receive your busy time but not supply it. If your work calendar lived in Outlook — which for most people is exactly where it lives — the one calendar that most needed to block out your evenings could not be read at all.
Signing in with a Microsoft account now works the same way Google does. Choose the calendar you want read, and Keeper.sh keeps its busy time flowing to every destination you have set up. A single account can be both ends at once: reading your work calendar and receiving your personal one, in the same connection.
Both providers now run through the same sync path rather than each carrying its own. That is invisible when things work, and it is why a fix to how repeating events or timezones are handled lands for Google and Outlook together instead of being written twice.
Read more about everything Keeper.sh does.