I like your solution, it is elegant but a bit slower. Conversion of date strings to integers is expensive, but it pays off in the sorting faze. Dates that are close together have 4 to 6 equal characters from the beginning of the string, and the for loop is also faster than map. I made a couple of benchmarks to verify that. The difference in speed is not significant.
I had to assume some date format to be able to produce working solution. It is easy to modify for different format.