"Creation date" is a predefined property that corresponds to the date of the first revision of a subject and is provided by Semantic MediaWiki. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
Creation date is a system property of SMW, available per default on every page in every namespace of the wiki. It contains the timestamp of the last modification of the article and has the type Date. Read more about it here.