For example, most of my entities have DateCreated and DateModified fields. The defaults to these are set to GetUtcDate() in SQL Server.
If I try and create an entity
You must set StoreGeneratedPattern for those properties to Identity for DateCreated and Computed for DataModified. It is available in designer. Once you do that you cannot modify those values in your application - only database can set those properties. I wrote about this some article because this feature had bug before VS2010 SP1 but there are reports that it still doesn't work sometimes.