in the jsreport.config,json I've turned off the preview with
"html-to-xlsx": {
"previewInExcelOnline": false
}
The Error occurs when trying to open in Excel. (something about the excel file need to be repaired)
I think an empty cell would work, But, it would leave that cell un-formatted.
The best would be to format the cell and leave it value blank.

I using the data-cell-type to format the Excel. The problem I am having is that if there is no date for that line.
I know I could wrap it with
{{#if date}}<td data-cell-type="date" data-cell-format-str="mm/dd/yy">...</td>{{else}}<td></td>{{/if}}

But, I was hoping there is another way.