I think I need to see it more clearly and test it myself, is there any sample file(s) you could send to firstname.lastname@example.org and also show screenshot of your settings, and how you want it to look like in the excel?
Have you tried using the Column --> Add and then specified the format?
I used the Columns tab under the Excel - Create tab to specify all the columns.
Excel Create Task Date Formats.zip
(17kb) downloaded 39 time(s).
Here's a zip file containing the input text file and several cases of outputs. The column in question is "date_sent", which is saved in the text file as a double-precision decimal. All Excel files are created using the Excel - Create task with output as Excel 2007.
Case 1 - Column format set to "@" (default)
Case 2 - Column format set to "#.##"
Case 3 - Column format set to "M/d/yyyy"
Case 4 - Column format set to "yyyy'-'MM'-'dd'T'HH':'mm':'ss"
Only in Case 2 is the date_sent column data presented as numeric. In all other cases, it's number formatted as text, so the recipient of the file has to manually convert the data to numeric and apply the date format to it.