Update writer documentation stating that DateTimeInterface implemento…

…rs are acceptable

docs/book/writer.md

@@ -213,9 +213,9 @@ Method | Description
 `setFeedLink()` | Set a URI to an XML feed, whether it is to the feed being generated, or an alternate URI pointing to the same feed but in a different format. At a minimum, it is recommended to include a link to the feed being generated so it has an identifiable final URI allowing a client to track its location changes without necessitating constant redirects. The parameters are the feed URI and type (one of "atom", "rss", or "rdf").
 `addAuthors()` | Sets the data for authors. The parameter is an array of array,s where each sub-array may contain the keys "name", "email", and "uri". The "uri" value is only applicable for Atom feeds, since RSS contains no facility to show it. For RSS 2.0, rendering will create two elements: an author element containing the email reference with the name in brackets, and a Dublin Core creator element only containing the name.
 `addAuthor()` | Sets the data for a single author following the same array format as described above for a single sub-array.
-`setDateCreated()` | Sets the date on which this feed was created. Generally only applicable to Atom, where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or a `DateTime` object.
-`setDateModified()` | Sets the date on which this feed was last modified. The expected parameter may be a UNIX timestamp or a `DateTime` object.
-`setLastBuildDate()` | Sets the date on which this feed was last build. The expected parameter may be a UNIX timestamp or a `DateTime` object. This will only be rendered for RSS 2.0 feeds, and is automatically rendered as the current date by default when not explicitly set.
+`setDateCreated()` | Sets the date on which this feed was created. Generally only applicable to Atom, where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or an object implementing `DateTimeInterface` such as `DateTime` or `DateTimeImmutable`.
+`setDateModified()` | Sets the date on which this feed was last modified. The expected parameter may be a UNIX timestamp or an object implementing `DateTimeInterface` such as `DateTime` or `DateTimeImmutable`.
+`setLastBuildDate()` | Sets the date on which this feed was last build. The expected parameter may be a UNIX timestamp or an object implementing `DateTimeInterface` such as `DateTime` or `DateTimeImmutable`. This will only be rendered for RSS 2.0 feeds, and is automatically rendered as the current date by default when not explicitly set.
 `setLanguage()` | Sets the language of the feed. This will be omitted unless set.
 `setGenerator()` | Allows the setting of a generator. The parameter should be an array containing the keys "name", "version", and "uri". If omitted a default generator will be added referencing `Zend\Feed\Writer`, the current zend-version version, and the Framework's URI.
 `setCopyright()` | Sets a copyright notice associated with the feed.
@@ -257,8 +257,8 @@ Method | Description
 `setLink()` | Set a URI to the HTML website containing the same or similar information as this entry (i.e. if the feed is from a blog, it should provide the blog article's URI where the HTML version of the entry can be read).
 `addAuthors()` | Sets the data for authors. The parameter is an array of array,s where each sub-array may contain the keys "name", "email", and "uri". The "uri" value is only applicable for Atom feeds, since RSS contains no facility to show it. For RSS 2.0, rendering will create two elements: an author element containing the email reference with the name in brackets, and a Dublin Core creator element only containing the name.
 `addAuthor()` | Sets the data for a single author following the same format as described above for a single sub-array.
-`setDateCreated()` | Sets the date on which this entry was created. Generally only applicable to Atom where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or a `DateTime` object. If omitted, the date used will be the current date and time.
-`setDateModified()` | Sets the date on which this entry was last modified. The expected parameter may be a UNIX timestamp or a `DateTime` object. If omitted, the date used will be the current date and time.
+`setDateCreated()` | Sets the date on which this entry was created. Generally only applicable to Atom where it represents the date the resource described by an Atom 1.0 document was created. The expected parameter may be a UNIX timestamp or an object implementing `DateTimeInterface` such as `DateTime` or `DateTimeImmutable`. If omitted, the date used will be the current date and time.
+`setDateModified()` | Sets the date on which this entry was last modified. The expected parameter may be a UNIX timestamp or an object implementing `DateTimeInterface` such as `DateTime` or `DateTimeImmutable`. If omitted, the date used will be the current date and time.
 `setCopyright()` | Sets a copyright notice associated with the entry.
 `addCategories()` | Accepts an array of categories for rendering, where each element is itself an array whose possible keys include "term", "label", and "scheme". The "term" is a typically a category name suitable for inclusion in a URI. The "label" may be a human readable category name supporting special characters (it is encoded during rendering) and is a required key. The "scheme" (called the domain in RSS) is optional but must be a valid URI.
 `addCategory()` | Sets the data for a single category following the same format as described above for a single sub-array.