Hi Fenella,
Thanks for your interest in Essential XlsIO.
You can set the cell in the worksheet as Date by using Value property. Please refer the following code snippet to achieve this:
//Inserting Date into the first worksheet
sheet.Range["A1"].Value = "12/05/2005";
sheet.Range["A2"].Value = "11/05/2005";
Here is the sample for your reference:
http://websamples.syncfusion.com/samples/XlsIO.Windows/F60933/main.htmKindly let me know if you have any other questions.
Thanks,
Melba