I always recommend a three step approach
That way you have full control and you can check the independent steps. My guess is that the value would be imported correctly as a string, but somehow the format you use is not correct (common mistakes are using mm (minutes) in stead of MM (month).
Please refer to https://docs.mendix.com/refguide9/parse-and-format-date-function-calls/ for more information on the subject.