This attribute accepts "true" or "false". With Utc="true" the time entered on a data form is converted and stored as Utc time when the record is saved.  When records are opened the Utc time is evaluated against local system settings to display the time in the local time zone.

When the attribute is absent DateTime values are stored using the local time.

For example, with the following element defining a WhenUpdated field on a News Article record - an article updated at 5:00 PM eastern time would appear to a user in the Central Time zone as being updated at 4:00 PM.
<Column Id="WhenUpdated" Name="Last Updated" Type="DateTime" Utc="true" Format="M/d/yyyy h:mm:ss tt" />