Swift full date with milliseconds

Updated for Swift 3

let d = Date()
let df = DateFormatter()
df.dateFormat = "y-MM-dd H:mm:ss.SSSS"

df.string(from: d) // -> "2016-11-17 17:51:15.1720"

When you have a Date d, you can get the formatted string using a NSDateFormatter. You can also use a formatter to turn a string date based on your format into a Date

See this chart for more on what dateFormat can do http://waracle.net/iphone-nsdateformatter-date-formatting-table/