the date-as-number is different on FM and Excel, so conversion to number isn't helpful and you would still need to format the column as date.
this may not help, but on occasion, I've had to export with a single quote before text (like numbers in a postal code) to prevent Excel from converting to a number and dropping leading 0's.
Is the problem the column width in Excel, I wonder? or something else?
Dates will also display hash marks if you attempt to display a date value outside the range of dates that Excel can handle (1/1/1900 through 12/31/9999).
(FileMaker dates can go back to 1/1/0001.)
I think, this is just a column width issue. Your dates are a bit wider than default column width. Try to expand a column were the dates are.
BTW, Beverly already suggested this as part of her answer.
I tested (FM14) with Save/Send Records As (Excel) and Export Records (Excel Workbooks). The latter allows "Apply current layout's data formatting to exported data". While help says checking this little "format" will give us the date format (if not as entered), I was not able to get that with Excel. Export Record (Comma separated) did preserve layout formatting of the date. That being said...
I also created a calculated TEXT field and made it to format the date in a way that I preferred for the export. In both types of "Excel" exports (as explained above), this field was text in Excel. Perhaps you need to do the same and create a field that is text calculation by equal to the date? It may be worth a test.