How can I set an NSDate object to midnight?

Question

I have an NSDate object and I want to set it to an arbitrary time (say, midnight) so that I can use the timeIntervalSince1970 function to retrieve data consistently without worrying about the time when the object is created.

I've tried using an NSCalendar and modifying its components by using some Objective-C methods, like this:

let date: NSDate = NSDate()
let cal: NSCalendar = NSCalendar(calendarIdentifier: NSGregorianCalendar)!

let components: NSDateComponents = cal.components(NSCalendarUnit./* a unit of time */CalendarUnit, fromDate: date)
let newDate: NSDate = cal.dateFromComponents(components)

The problem with the above method is that you can only set one unit of time (/* a unit of time */), so you could only have one of the following be accurate:

1. Day
2. Month
3. Year
4. Hours
5. Minutes
6. Seconds

Is there a way to set hours, minutes, and seconds at the same time and retain the date (day/month/year)?

Answer 1

Your statement

The problem with the above method is that you can only set one unit of time ...

is not correct. NSCalendarUnit conforms to the RawOptionSetType protocol which inherits from BitwiseOperationsType. This means that the options can be bitwise combined with & and |.

In Swift 2 (Xcode 7) this was changed again to be an OptionSetType which offers a set-like interface, see for example Error combining NSCalendarUnit with OR (pipe) in Swift 2.0.

Therefore the following compiles and works in iOS 7 and iOS 8:

let date = NSDate()
let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!

// Swift 1.2:
let components = cal.components(.CalendarUnitDay | .CalendarUnitMonth | .CalendarUnitYear, fromDate: date)
// Swift 2:
let components = cal.components([.Day , .Month, .Year ], fromDate: date)

let newDate = cal.dateFromComponents(components)

(Note that I have omitted the type annotations for the variables, the Swift compiler infers the type automatically from the expression on the right hand side of the assignments.)

Determining the start of the given day (midnight) can also done with the rangeOfUnit() method (iOS 7 and iOS 8):

let date = NSDate()
let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
var newDate : NSDate?

// Swift 1.2:
cal.rangeOfUnit(.CalendarUnitDay, startDate: &newDate, interval: nil, forDate: date)
// Swift 2:
cal.rangeOfUnit(.Day, startDate: &newDate, interval: nil, forDate: date)

If your deployment target is iOS 8 then it is even simpler:

let date = NSDate()
let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
let newDate = cal.startOfDayForDate(date)

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Update for Swift 3 (Xcode 8):

let date = Date()
let cal = Calendar(identifier: .gregorian)
let newDate = cal.startOfDay(for: date)