a) You shouldn't be using datenum. They're completely obsolete and everything you could do with them, you can do with datetime, and more.
b) The way a number is stored and the way a number is displayed are two completely different things. You can change the way matlab displays numbers at the command window wth the format command. Note that this will in no way affect the actual number, nor will it affects the way it is exported in a text or excel file. Again, this only affects the display in the command window
>> dn = now;
>> format short
>> format long
>> format longg