2006-2017 (Windows Vista)
2009-present (Windows 7)
In 2012, Hotmail Calendar became simply Calendar as part of Windows 8 applications. It is still used online.
2015-present (Windows 10)
This is the icon used on the Windows 10 app
Outlook Calendar (Windows 10 Mobile)
On Windows 10 Mobile devices, the Outlook Calendar app replaced the Calendar app to further attach it to the Outlook Mail. This is the only case of the Outlook Calendar logo. Other versions use the regular Outlook Mail logo.