How do I sort a swift array containing instances of NSManagedObject subclass by an attribute value (date)

This is partly an issue with the Swift compiler not giving you a helpful error. The real issue is that NSDate can't be compared with < directly. Instead, you can use NSDate's compare method, like so:

days.sort({ $0.date.compare($1.date) == NSComparisonResult.OrderedAscending })

Alternatively, you could extend NSDate to implement the Comparable protocol so that it can be compared with < (and <=, >, >=, ==):

public func <(a: NSDate, b: NSDate) -> Bool {    return a.compare(b) == NSComparisonResult.OrderedAscending}public func ==(a: NSDate, b: NSDate) -> Bool {    return a.compare(b) == NSComparisonResult.OrderedSame}extension NSDate: Comparable { }

Note: You only need to implement < and == and shown above, then rest of the operators <=, >, etc. will be provided by the standard library.

With that in place, your original sort function should work just fine:

days.sort({ $0.date < $1.date })

In Swift 3, dates are now directly comparable:

let aDate = Date()let bDate = Date()if aDate < bDate {    print("ok")}

Old swift:An alternative would be to sort on the timeIntervalSince1970 property of the date object, which is directly comparable.

days.sort({$0.date.timeIntervalSince1970 < $1.date.timeIntervalSince1970})

In swift2.1 use this lines of code

array.sortInPlace({ $0.date.compare($1.date) == NSComparisonResult.OrderedAscending })