// First I output 2018-12-01 00:00:00 as a date
currentDate = Date.today().toStartofMonth();

// Outputs 2018-12-01 as a string
firstDayOfCurrentMonth = currentDate.addMonths(i).toStartofMonth();
String currentDateString = String.valueOf(firstDayOfCurrentMonth).removeEnd('00:00:00');

// Outputs 2018-12-01 00:00:00
Date.valueOf(currentDateString);

// Outputs 2018-12-01 00:00:00
Date.valueOf(currentDateString.removeEnd(' 00:00:00'))

Date.parse(currentDateString)
// Throws error with Invalid Date : 2018-12-01