NSInvalidArgumentException, reason: 'Invalid type in JSON write (__NSDate)'

Convert NSDate to NSString and try to encode.

- (NSString *) changeDateToDateString :(NSDate *) date {

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setTimeZone:[NSTimeZone localTimeZone]];
    NSLocale *locale = [NSLocale currentLocale];
    NSString *dateFormat = [NSDateFormatter dateFormatFromTemplate:@"hh mm" options:0 locale:locale];
    [dateFormatter setDateFormat:dateFormat];
    [dateFormatter setLocale:locale];
    NSString *dateString = [dateFormatter stringFromDate:date];
    return dateString;
}

FIrst store your data in NSString. And then convert your string to NSDate.

You can refer SO:

Converting NSString to NSDate (and back again)

NSString to NSDate conversion problem

How to convert NSString to NSDate using NSDateFormatter?

As noted, you must first convert your NSDate to an NSString. It's not immediately clear, however, which format the date should be represented in. The answer can be found here: "JSON itself does not specify how dates should be represented, but Javascript does" -- ISO8601.

Here is an ISO8601 conversion method from a helper category for NSDate, courtesy Erica Sadun:

- (NSString *)ISO8601 {
    struct tm time;
    time_t interval = [self timeIntervalSince1970];
    gmtime_r(&interval, &time);
    char *x = calloc(1, 21);
    strftime_l(x, 20, "%FT%TZ", &time, gmtlocale);
    NSString *string = [NSString stringWithUTF8String:x];
    free(x);
    return string;
}

If you get an ISO8601 string back in a JSON payload and want to convert it to an NSDate, use this class method for NSDate:

+ (NSDate *)dateFromISO8601:(NSString *)string {
    if(!string) return nil;
    if (![string isKindOfClass:[NSString class]]) return nil;
    struct tm time;
    strptime_l([string UTF8String], "%FT%TZ", &time, gmtlocale);
    return [NSDate dateWithTimeIntervalSince1970:timegm(&time)];
}