I've found following issues related to DateDiff:
a. When I've exported data to excel there is no information that process has been completed. Imho there shall be a popup with link to exported file [I know - I had to provide path first]
b. Default option is "resize column to fit window". I have changed this to "fit to data" then exported to excel.
Although the way information is displayed has been changed, result of export is the same in both cases.