Convert a mysql date to NSDate then check how many days between - swift

Question

I'm having problems with NSDate in Swift.
I JSON a date formatted in Y-m-d from my database. This is stored as a string called survey_date
I then convert the string to a NSDate using dateFormatter
I want to work out the amount of days between the survey_date and NOW
Error in my script is endDate is ambiguous expression without more context.
Here is my script:
var survey_date = prefs.valueForKey("SURVEY_DATE") as! String
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd"
var date = dateFormatter.dateFromString(survey_date) as NSDate!
let outputDate = dateFormatter.stringFromDate(date)
println(outputDate)
let start = outputDate
let end = NSDate()
let dateFormatter2 = NSDateFormatter()
dateFormatter2.dateFormat = "yyyy-MM-dd"
let startDate:NSDate = dateFormatter2.dateFromString(start)!
let endDate:NSDate = dateFormatter2.dateFromString(end)!
let cal = NSCalendar.currentCalendar()
let unit:NSCalendarUnit = .CalendarUnitDay
let components = cal.components(unit, fromDate: startDate, toDate: endDate, options: nil)
println(components)
Secondly, I also want to be able to convert the date when possible to a different format - for example d m, Y. This is pretty simple to do in PHP but I can't find a way of doing it in Swift? I don't want to have to send extra data through JSON (i.e. create a $formatted_date) but it looks like this is my only option (which I know is messy and I will get told off for!)

Answer 1

You get a compiler error in
let endDate:NSDate = dateFormatter2.dateFromString(end)!
because dateFromString() is called with a date instead of a string,
which makes no sense.
Your computation is a bit too complicated, you can shorten it to
let survey_date = "2015-05-01"
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd"
if let startDate = dateFormatter.dateFromString(survey_date) {
let endDate = NSDate()
let cal = NSCalendar.currentCalendar()
let components = cal.components(.CalendarUnitDay, fromDate: startDate, toDate: endDate, options: nil)
let days = components.day
println(days)
} else {
println("invalid input")
}
If you want to convert the date into a different format then
(as Leo already said in a comment), use stringFromDate() with
a second date formatter, e.g.
let dateFormatter2 = NSDateFormatter()
dateFormatter2.dateFormat = "d MM, Y"
let formattedStartDate = dateFormatter2.stringFromDate(startDate)
println(formattedStartDate)

Answer 2

Here's something you may find handy. I was working on an app that did a lot of date calculations, and therefore was making lots of calls to NSCalendar's components(_:fromDate:toDate:options:) method. This doesn't feel terribly Swift-like, so I overloaded the - operator to do date subtractions:
func -(lhs: NSDate, rhs: NSDate) -> NSDateComponents
{
let components: NSCalendarUnit =
.CalendarUnitSecond |
.CalendarUnitMinute |
.CalendarUnitHour |
.CalendarUnitDay
return NSCalendar.currentCalendar().components(components,
fromDate: rhs,
toDate: lhs,
options: nil)
}
This lets you find the difference between any two dates/times by simple subtraction. Here's a quick example:
// MySQL date formatter
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd"
// Let's create a couple of dates that we know are one day apart
let christmas2015 = dateFormatter.dateFromString("2015-12-25")!
let christmasEve2015 = dateFormatter.dateFromString("2015-12-24")!
let difference = christmas2015 - christmasEve2015
let daysBetween = difference.day // this value is 1