store the value as a date time value the date part will default to 01 Jan 1900, but that shoudln't matter.
after its stored as a datetime value then you can use the datetime functions such as datediff
but then again
since this database is in a work flow, each step has a Date Completed and Date Received
there is no reason why you cannot store the datetime a process started and a datetime when the process finished
the process doesn't need to be autofilled as defacto when the proceeding process completes then its available for the next process which depending on the production may happen immediately afterwards or may happen at a time int he future.