date_units.swift

extension Date {
    
    func add(_ unit: Calendar.Component, value: Int) -> Date {
        return Calendar.current.date(byAdding: unit, value: value, to: self)!
    }
    
    func add(year: Int) -> Date {
        return add(.year, value: year)
    }
    
    func add(month: Int) -> Date {
        return add(.month, value: month)
    }
    
    func add(day: Int) -> Date {
        return add(.day, value: day)
    }
    
    func add(minute: Int) -> Date {
        return add(.minute, value: minute)
    }
    
    func add(hour: Int) -> Date {
        return add(.hour, value: hour)
    }
    
}

let startDate = Date() // Sep 17, 2018 at 7:52 PM
let plusOneHour = startDate.add(hour: 1) // Sep 17, 2018 at 8:52 PM
let minusYear = startDate.add(year: -1) // Sep 17, 2017 at 7:52 PM