Rounding of date and time

DateRounding.swift

import Foundation

extension Date {
    
    /// Returns date where **-component** is rounded to its closest
    /// multiple of **-amount**. Warning: month and day start at 1
    /// so round(to: 6, .month) will either return month 1 or 7!
    func round(to amount: Int, _ component: Calendar.Component) -> Date {
        let cal = Calendar.current
        var value = cal.component(component, from: self)
        
        if [.month, .day].contains(component) {
            // Months and days start at 1, time/year starts at 0
            value -= 1
        }
        
        // Compute nearest multiple of amount
        let fraction = Double(value) / Double(amount)
        let roundedValue = Int(fraction.rounded()) * amount
        let newDate = cal.date(byAdding: component, value: roundedValue - value, to: self)!
        
        return newDate.floorAllComponents(before: component)
    }
    
    /// Returns date where all components before paramater are set to
    /// their beginning value; day and month to 1 and everything else
    /// to 0
    func floorAllComponents(before component: Calendar.Component) -> Date {
        // All components to round ordered by length
        let components = [Calendar.Component.year, .month, .day, .hour, .minute, .second, .nanosecond]
        
        guard let index = components.firstIndex(of: component) else {
            assertionFailure("Unsupported component")
            return self
        }
        
        let cal = Calendar.current
        var date = self
        
        components.suffix(from: index + 1).reversed().forEach { roundComponent in
            var value = cal.component(roundComponent, from: date) * -1
            if [.month, .day].contains(roundComponent) {
                // Months and days start at 1, time/year starts at 0
                value += 1
            }
            date = cal.date(byAdding: roundComponent, value: value, to: date)!
        }
        
        return date
    }
    
    init?(_ string: String, format: String) {
        guard let date = DateFormatter(format).date(from: string) else {
            return nil
        }
        self = date
    }
}

DateRoundingTests.swift

extension DateFormatter {
    convenience init(_ dateFormat: String) {
        self.init()
        self.dateFormat = dateFormat
        timeZone = Calendar.current.timeZone
        locale = Calendar.current.locale
    }
    
    convenience init(date: Style, time: Style) {
        self.init()
        dateStyle = date
        timeStyle = time
        timeZone = Calendar.current.timeZone
        locale = Calendar.current.locale
    }
}

let f = DateFormatter("yyyy-MM-dd HH:mm:ss")
let date = f.date(from: "2018-2-5 16:30:15")!

var floorDates = [Date]()
floorDates.append(date.floorAllComponents(before: .second)) // 2018-02-05 16:30:15
floorDates.append(date.floorAllComponents(before: .minute)) // 2018-02-05 16:30:00
floorDates.append(date.floorAllComponents(before: .hour))   // 2018-02-05 16:00:00
floorDates.append(date.floorAllComponents(before: .day))    // 2018-02-05 00:00:00
floorDates.append(date.floorAllComponents(before: .month))  // 2018-02-01 00:00:00
floorDates.append(date.floorAllComponents(before: .year))   // 2018-01-01 00:00:00

print("Floor")
floorDates.forEach { date in
    print(f.string(from: date))
}

var roundDates = [Date]()
roundDates.append(f.date(from: "2018-2-1 16:30:15")!.round(to: 30, .second)) // 2018-02-01 16:30:30
roundDates.append(f.date(from: "2018-2-1 16:30:14")!.round(to: 30, .second)) // 2018-02-01 16:30:00
roundDates.append(f.date(from: "2018-2-1 16:30:15")!.round(to: 20, .minute)) // 2018-02-01 16:40:00
roundDates.append(f.date(from: "2018-2-1 16:29:15")!.round(to: 20, .minute)) // 2018-02-01 16:20:00
roundDates.append(f.date(from: "2018-3-1 16:30:15")!.round(to: 6,  .month))  // 2018-01-01 00:00:00
roundDates.append(f.date(from: "2018-4-1 16:30:15")!.round(to: 6,  .month))  // 2018-07-01 00:00:00
roundDates.append(f.date(from: "2018-2-5 16:30:15")!.round(to: 10, .day))    // 2018-01-01 00:00:00
roundDates.append(f.date(from: "2018-2-6 16:30:15")!.round(to: 10, .day))    // 2017-11-01 00:00:00
roundDates.append(f.date(from: "2018-2-1 16:30:15")!.round(to: 10, .year))   // 2020-01-01 00:00:00
roundDates.append(f.date(from: "2018-2-1 16:30:15")!.round(to: 9,  .hour))   // 2018-02-01 18:00:00

print("Round")
roundDates.forEach { date in
    print(f.string(from: date))
}