The datetime I'm writing to Excel always get rounded up to the next day:
workSheet.Cells[0, 0] = new Cell(DateTime.Now, new CellFormat(CellFormatType.DateTime, @"HH:mm:ss"));
In the output Excel file the cell gets this value: 29/09/2013 00:00:00
The DateTime.Now from this example is 28/09/2013 19:42:23
I ended up passing the cell value as a string instead of as a DateTime:
workSheet.Cells[0, 0] = new Cell(DateTime.Now.ToString(@"HH:mm:ss:ff"),
new CellFormat(CellFormatType.DateTime, @"HH:mm:ss"));