I suppose this is on SQL Server.
If you expect to be able to add some offset when altering the column, then (I’m almost sure) that is not possible. The database will assign an offset of ’00:00′ because the offset is not included in the datetime type, so your data doesn’t contain an offset.
You would have to alter the column, and then update the data, for which you could use the <a href=”http://msdn.microsoft.com/en-us/library/bb677244.aspx”>SWITCHOFFSET function</a> (but that would be a long operation on a table that size).
Use TOSYSDATETIMEOFFSET() FUNCTION