ICalendar

iCalendar, or ICS, is a standardized format for storing and transmitting calendar data, including scheduled events and "to-do" lists. People often associate it with Apple's iCal program (now just "Calendar"), but it is a standardized format with an official RFC document (RFC 5545) and the format is supported by many calendar programs including Google Calendar and Mozilla Lightning (plugin for Thunderbird). Microsoft Outlook does the typical Microsoft thing of supporting something vaguely resembling the standard but full of quirky incompatibilities.