PostgreSQL

A free open-source database server which endeavours to conform to the SQL standard and which is extensible with server-side functions and provides high performance, particularly to write-heavy applications.

http://www.postgresql.org/

PostgreSQL is:
 * highly conformant to the SQL specification
 * supports transactions wrapping database changes (CREATE TABLE, etc)
 * has a very full-featured set of types and functions related for time and date handling
 * allows in-database function extensions in a variety of languages

DAViCal has implicit dependencies on some of these features.