Thanks for your posting and using Aspose.Cells.
Please download and use the latest version: Aspose.Cells
for .NET v184.108.40.206 it is working fine with your code.
I have tested this issue with the following sample code. I have attached the source excel file used in this code and output excel file generated by it for your reference. I have also shown the screenshot which shows that cell A5 is now having Date format highlighted with red circle.
string filePath = @“D:\Downloads\source.xlsx”;
Workbook workbook = new Workbook(filePath);
Worksheet _workSheet = workbook.Worksheets;
Aspose.Cells.Style st = _workSheet.Cells[“A5”].GetStyle();
st.Number = 14;
workbook.Save(filePath + “.out.xlsx”);