Why/how does the text from the Notepad file get converted to a number anyway? Do you need to? Why can't you just leave it as a string? If you need it as a number internally in your MATLAB program, and converted it with datenum, I still think you could keep the original string for later export to Excel. I think that's what I'd recommend.
Alternatively, if you have date numbers, you could convert them to a string with datestr() and then stick those strings into a cell array and write the cell array out.
Another option is to make an Excel workbook up in advance with the proper cell formatting. Then you just call xlswrite to that existing workbook (don't delete it first!) and the numbers will appear with the predefined format you set up.
Finally, the most flexible and powerful, but most complicated, is to use ActiveX to directly control Excel from MATLAB. If you want to go that route, I've attached a demo. Not for setting cell formatting, but just ActiveX in general. To figure out what you need to do you need to record a macro in Excel where you format the cells to the date appearance that you like, then look at the macro that got recorded, and transfer those commands over to your MATLAB code. I use ActiveX all the time, but it has a shallow learning curve (amount learned vs. time is low), not a steep learning curve (amount learned vs. time is high). But if you're up for it, give it a try - it's worth learning.
