Here is your problem: DateField.dateToString's first parameter is supposed to be a date. It then takes that date and returns a string using the second parameter as a format string.
It looks like you're trying to convert the string to a date (the other way around) so you can get the seconds from it and put it in the text area. The DateField control has a selectedDate parameter that will give you the date you need. Then you just run this code to put it in the text area:
testTextArea.text = myDateField.selectedDate.getSeconds().toString();