The UpdateEntitiesDirectly method is virtual, so you could override it and first check the values in the entity specified, or you could use the event OnUpdateEntitiesDirectly, but that's a little later in the pipeline as it already has created the query (so you need to change the parameter values in that case).
Alternatively you could override OnSetValue in a partial class of the entity, and check the value there if it's a datetime and if so convert it to a UTC datetime
And yet another alternative is to define a typeconverter on the datetime field which does the datetime <->UTC datetime conversion for you. This type converter is built into the framework (and shipped with the designer) so it might be the best option (and you don't need to convert the value manually)
