DataSetHelper.CreateWorkbook uses 1904 Date system

```
What steps will reproduce the problem?
1. Create Dataset
2. Save dataset using ExcelLibrary.DataSetHelper.CreateWorkbook
3. Open workbook in Excel

What is the expected output? What do you see instead?
Standard excel behaviour is to create workbooks not using the 1904 date system,
workbooks using this library do use the 1904 date system (in Excel options you
can see this). Other than that, it's perfect.

What version of the product are you using? On what operating system?
Windows XP with Office 2007

Please provide any additional information below.

```

Original issue reported on code.google.com by `garethte...@gmail.com` on 26 Oct 2010 at 2:06

Start at DataSetHelper.CreateWorkbook, following how the generated workbook's date system is written. Reproduce the issue by creating and opening a dataset workbook in Excel, then verify that the workbook uses the standard date system rather than 1904. The payload names no test file, so identify the project's existing test location before adding regression coverage.

Written by the indexing model from the issue text.