Parse JSON date from .net WebService in Objective-C

In my iOS class @FHNW I set the students a task for creating a CRUD application. The backend is an existing .net REST WebService with JSON. Unfourtanatly Microsoft has decided to create their own date formate instead of using a standard like RFC1123. Therefore the solution to display the date correctly or parsing an NSDate is not strait forward.

You can parse the date the the following snipped:

+ (NSDate *)mfDateFromDotNetJSONString:(NSString *)string {
    static NSRegularExpression *dateRegEx = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        dateRegEx = [[NSRegularExpression alloc] initWithPattern:@"^\\/date\\((-?\\d++)(?:([+-])(\\d{2})(\\d{2}))?\\)\\/$" options:NSRegularExpressionCaseInsensitive error:nil];
    });
    NSTextCheckingResult *regexResult = [dateRegEx firstMatchInString:string options:0 range:NSMakeRange(0, [string length])];

    if (regexResult) {
        // milliseconds
        NSTimeInterval seconds = [[string substringWithRange:[regexResult rangeAtIndex:1]] doubleValue] / 1000.0;
        // timezone offset
        if ([regexResult rangeAtIndex:2].location != NSNotFound) {
            NSString *sign = [string substringWithRange:[regexResult rangeAtIndex:2]];
            // hours
            seconds += [[NSString stringWithFormat:@"%@%@", sign, [string substringWithRange:[regexResult rangeAtIndex:3]]] doubleValue] * 60.0 * 60.0;
            // minutes
            seconds += [[NSString stringWithFormat:@"%@%@", sign, [string substringWithRange:[regexResult rangeAtIndex:4]]] doubleValue] * 60.0;
        }

        return [NSDate dateWithTimeIntervalSince1970:seconds];
    }
    return nil;
}

This solution is copied from stack overflow.

To produce a .net WebService compatible date string again the solution is a little bit simpler. The important parts are caring about second/millisecond base and the time zones:

+ (NSString*)dotNetJSONStringFromDate:(NSDate*)date
{
    NSDateFormatter* formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"ZZZ"];
    
    NSString* timezone = [formatter stringFromDate:date];
    
    int timezoneOffset = [timezone intValue] * 36;
    NSTimeInterval timeInterval = [date timeIntervalSince1970];
    
    unsigned long long time=((timeInterval - timezoneOffset) * 1000);
    NSString* dateTimeTosend= [NSString stringWithFormat:@"/Date(%lld%@)/", time, timezone];
    
    return dateTimeTosend;
}