When does the Christian Sabbath begin and end?

There was an interesting discussion recently on the RB Yahoo list concering the proper timing of the Christian Sabbath.  Is it from sundown Saturday to sundown Sunday or is it from sunup Sunday to sunup Monday?  One post included this link to a blog article by Old School Presbyterian Andrew Webb arguing for the sunup Sunday to sunup Monday view (borrowing from the Puritan Thomas Vincent).  I'm still thinking about the arguments.

JTR