To test Nullable types use the HasValue which is a bool like this.
if ( Table1.prevDay.HasValue )
If a post answers your question, please click "Mark As Answer" on that post and "Mark as Helpful".
NOTE: If I ask for code, please provide something that I can drop directly into a project and run (including XAML), or an actual application project. I'm trying to help a lot of people, so I don't have time to figure out weird snippets with undefined objects
and unknown namespaces.