Sorted_by.swift

import Foundation

class Student {
    var enrolledAt:Date = Date()
    
    init(enrolledAt:String) {
        let dateFormatter = ISO8601DateFormatter()
        dateFormatter.formatOptions = .withFullDate
        self.enrolledAt = dateFormatter.date(from: enrolledAt)!
    }
    init() {
        self.enrolledAt = Date()
    }
}

let students:[Student] = [
    Student(),
    Student(enrolledAt: "2018-02-02"),
    Student(enrolledAt: "2017-01-01"),
    Student(enrolledAt: "2017-12-31")
]

students.sorted(by: {$0.enrolledAt > $1.enrolledAt})

students.sorted(by: {$0.enrolledAt < $1.enrolledAt})