Hi Sean,
You can format cells as time values by using the following code snippets:
[C#]
sheet.Range["C2"].NumberFormat = "h:mm:ss";
Here is the sample for your reference:
http://websamples.syncfusion.com/samples/XlsIO.Windows/63089/main.htmKindly let me know if you have any other questions.
Thanks,
Melba