First of all I created an input parameter "n", to define all "old enough" records as those records where "date of last visit" happened more than "n" months ago with respect to today's date.
Secondly, I eliminated all records with a missing value in the field "date of last vist".
Then I started working with data of type datetime.
String DATE_FORMAT = "dd-MM-yyyy";
SimpleDateFormat sdf = new SimpleDateFormat(DATE_FORMAT);
Calendar today = Calendar.getInstance(); // today
2. Today's date as well as "date of last visit" was converted into Datetime type with a "String to Date/Time" node
3. Node "Time difference" outputs the difference in months between the two dates as an integer. Here you need to enable "Use second column" and to select the difference granularity (months in this case).
That's as much as it took to implement a date comparison and to filter out only the "old enough" dates.