I agree with Mauricio, us a separate field. But I would also recommend keeping in mind the pdate field type that is explained in the example Solr schema (see below). Especially if you will have missing date data on documents and will need to sort those first/last.
`Note:
These should only be used for compatibility with existing indexes (created with older Solr versions)
or if "sortMissingFirst" or "sortMissingLast" functionality is needed. Use Trie based fields instead.
Plain numeric field types that store and index the text
value verbatim (and hence don't support range queries, since the
lexicographic ordering isn't equal to the numeric ordering)
<fieldType name="pdate" class="solr.DateField" sortMissingLast="true" omitNorms="true"/>`