Format

The Format attribute accepts a string value that uses valid .NET specifiers for that datatype. 

Please refer to the Microsoft Developer Network for documentation regarding Date / Time specifiers and Custom Numeric specifiers.

Date / Time field example:
<Column Id="WhenCreated" Name="Created" Type="DateTime" Utc="true" ReadOnly="true" NoOverride="ReadOnly" Format="M/d/yyyy h:mm:ss tt" DedupIgnore="true"/>