At Arizona Archives Online we normalize undated material by taking a best guess as to its creation.  We often use the birth and death dates of the creator or a large date span, such as an entire century, for the normalized dates.  This is based on the idea that while the material is undated, as archivists, we can take an educated guess about its creation.

So our fields look like:
<unitdate encodinganalog="245$f" type="inclusive" normal="1800/1900">Undated.</unitdate>

Hope this helps,

Catalina Oyler
Project Archivist
Arizona Archives Online