How do I prefill a DataGrid cell with the current date/time

I have seen code that suggests setting the Default property of the DataColumn in the corresponding DataTable to DateTime.Now. This solution uses the date/time at the moment of that call. How do I insert the "real" current date/time? Is it possible to run "code" to supply a value? I have the column set as readonly, and as part of the primary key. I have not been able to find another way to fill the field when a new row is to be added in the DataGrid. Is there an event that is triggered when a new row is to be added, and can I add a handler? Thanks.

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