It's so funny. 2005-09-06 is treated as subtraction thus the result is 1990. You could also do datenum(2005,9,6) where the input arguments are date vector. See datevec().
If you have the string in numeric format only (as in your example), in the order yyyy-mm-dd, then you can compare strings directly to get date order.
If you want to convert it to another format, then you must remember to specify the fields, if you use numeric-only notation (your example could be yyyy-mm-dd or yyyy-dd-mm)
You cannot compare two dates in the string format yyyy-mm-dd directly, but you can use SORT to sort them, because then the chronological order equals the alphabetical order.
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