Events will be returned if the start or end date is contained within the
range (inclusive), or if the event starts before and end after the range.
If a start date is not specified or of a valid format, then the start
of the range will default to the current time and date of the server.
If an end date is not specified or of a valid format, the the end of
the range will default to the current time and date of the server,
plus 20 years.
Note that this function makes use of UNIX timestamps. This might be a
problem for events on, during, or after January the 29th, 2038.
See http://en.wikipedia.org/wiki/Unix_time#Representing_the_number