If you have R2014b or later, you can use readtable() to read the xls file in a way that the dates will appear as datetime objects that can then be sorted.
If not then you can use xlsread(). If the dates are stored in Excel date format then what you receive in MATLAB will be something you can sort(), even though it will not be exactly in MATLAB datenum format. If the dates are stored in strings in Excel then you can use datenum() to convert them to datenum format, which you can then sort.