So I was debugging some code I had written to perform a move/copy of a document within SharePoint and noticed none of the columns that were setup in SharePoint as DateTime were getting populated. Long story short, it turned out that our Upload Web Service is expecting metadata of a DateTime type to be in the UTC format (yyyy'-'MM'-'ddTHH':'mm':'ss'Z') which is a cross between the "s" and "u" built-in .Net DateTime.ToString() formatters. So I used:
[DateTimeObject].ToString("s") + "Z"