ActivityTimeline add-on allows integrating tasks on a timeline or custom events with third-party applications or third-party calendars such as Microsoft Outlook, Google Calendar, and Apple iCal. 

For more information refer to the following sections:

• Integrating with Google Calendar (v.5.0)
• Integrating with Outlook Calendar (v.5.0)
• Integrating with Apple iCal (v.5.0)