Sync your outlook calender to a gmail og google apps account

Download and install Outlook Google Calendar Sync

Disable warning of programmatic access in Outlook 2013:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\15.0\Outlook\Security]
"ObjectModelGuard"=dword:00000002

[HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\outlook\security]
"adminsecuritymode"=dword:00000003
"promptoomsend"=dword:00000002