date(from string: String) always returns nil - objective-c

Question

I have a NSDate on the iPhone App, that I need on the corresponding Watch App to do calculations, I don t even need to show it in a local format. Somehow it is working not really good if I transfer NSDate objects, I read that it s better to use Strings for transfer.
So I format my NSDate to a NSString. I tried different ways, and on the Watch side if I want to reverse it, I obviously use the same format. But it always returns nil and I don t understand why.
My code on the iPhone side:
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:#"dd/MM/yyyy HH:mm"];
NSString *dateString = [formatter stringFromDate:myDateObject];
I also tried:
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setFormatterBehavior:NSDateFormatterBehavior10_4];
[formatter setDateStyle:NSDateFormatterShortStyle];
[formatter setTimeStyle:NSDateFormatterShortStyle];
NSString *dateString = [formatter stringFromDate:myDateObject];
and I tried with:
formatter.locale = [NSLocale currentLocale];
On the Watch side I need to reverse my String to a date object:
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "dd/MM/yyyy HH:mm"
let date = dateFormatter.date (from: result["dateString"] as! String)
and:
let dateFormatter = DateFormatter()
ddateFormatter.formatterBehavior = .behavior10_4
dateFormatter.locale = .current
ateFormatter.dateStyle = .short
dateFormatter.timeStyle = .short
let date = dateFormatter.date (from: result["dateString"] as! String)
and with
dateFormatter.locale = .current
when I mouseover
result["dateString"]
I See a date string like
12/17/20, 12:27 PM
but the date(from string: String) always returns nil.
I also tried to set the format to fit like:
dateformat = "MM/dd/yyyy, h:mm a"
but nothing works. As I said, I don t need a specific localized format, I just need day and time, a date object to calculate with. What can I do?
Thanks for help!

Answer 1

First, make your date format match your input
A input like "12/17/20, 12:27 PM" suggests a format of something like MM/dd/yy, h:m a
Next, remove any possible issues the user's current locale may introduce by using something like dateFormatter.locale = Locale(identifier: "en_US_POSIX"), unless you can guarantee the locale format, this should suffice for most cases
For example
let dateString = "12/17/20, 12:27 PM"
let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "en_US_POSIX")
dateFormatter.dateFormat = "MM/dd/yy, h:m a"
let date = dateFormatter.date (from: dateString)
will generates 2020-12-17 01:27:00 +0000 on my PC (and could change depending on your system, which is why, it's always important to use a known DateFormatter to debug dates - removes possible confusion)

Answer 2

A great way to serialise dates is to use timeIntervalSinceReferenceDate which returns a double value. So on the iPhone side you'd have
double t = [myDateObject timeIntervalSinceReferenceDate];
Note that formally this will return TimeInterval but that is just a double and easy to cast.
The double you can either use as is or convert to string - this depends on how you transmit it. You have e.g. -setDouble:forKey and -doubleForKey with NSUserDefaults where you can use it as is as a double or you can print it to string using something like
[NSString stringWithFormat:#"%.20f", t]
This produces 20 digits after the decimal separator, which is overkill but you won't loose anything at least.
Once you have read or converted the double on the watch side you can of simply use NSDate's
init(timeIntervalSinceReferenceDate: TimeInterval)
to convert the double back to a date.