DateTime::Set is a module for datetime sets. It can be used to handle two
different types of sets.

The first is a fixed set of predefined datetime objects. For example, if we
wanted to create a set of datetimes containing the birthdays of people in our
family for the current year.

The second type of set that it can handle is one based on a recurrence, such as
"every Wednesday", or "noon on the 15th day of every month". This type of set
can have fixed starting and ending datetimes, but neither is required. So our
"every Wednesday set" could be "every Wednesday from the beginning of time
until the end of time", or "every Wednesday after 2003-03-05 until the end of
time", or "every Wednesday between 2003-03-05 and 2004-01-07".

This module also supports set math operations, so you do things like create a
new set from the union or difference of two sets, check whether a datetime is a
member of a given set, etc.

This is different from a DateTime::Span, which handles a continuous range as
opposed to individual datetime points. There is also a module DateTime::SpanSet
to handle sets of spans.