ToDateTime requires the value to be in the form YYYY-MM-DDThh:mm:ss[Z|(+|-)hh:mm]. Since your string doesn't look like that, you'll need to do string manipulation to pull out the year, month, day, etc and use the MakeDateTime function.
You need to be certain of the format of the response; if the value comes back as "2019/10/2" the expression above will cause an error b/c when the string is split by a space there is only one entry. Could you use a lambda with your lex action to force the response into a specific format?