Either however you created the text file and/or imported it, you ended up with the datenum integer portion of the date/time information (presuming there was time also originally). We'd have to know the rest of the process to be able to help regarding avoiding that.
But, given where you are, just convert the datenum to datetime via
>> t=[727168
727199
727230
727258
727289
727319
727350];
>> datetime(t,'ConvertFrom','datenum')
ans =
7×1 datetime array
01-Dec-1990 00:00:00
01-Jan-1991 00:00:00
01-Feb-1991 00:00:00
01-Mar-1991 00:00:00
01-Apr-1991 00:00:00
01-May-1991 00:00:00
01-Jun-1991 00:00:00
>>