You used datevec() which returns numeric contents not strings.
>> datevec(a,'mm/dd/yy HH:MM')
ans =
2010 7 17 20 36 0
You must have used datestr() to get the string; in that case use the format that you want
>> datestr(datevec(a,'mm/dd/yy HH:MM'), 'mm/dd/yy HH:MM')
ans =
07/17/10 20:36