Day of year from date string: weired behaviour for 01.01.2020

Question

I have a string with the date as xx.yy.2020 from a series of measurements during the year. I want to get an NSDate object and from that weekday and day of the year. My code:

           // setting parameters for testing:
        tag = 1;
        monat = 1;
        jahr = 2020;
        
        // Calendar object: TimeZone is Europe/Zurich. 
        NSCalendar *tagcalendar = [[NSCalendar alloc]  initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
        [tagcalendar setTimeZone:[NSTimeZone localTimeZone]];
        [tagcalendar setLocale:[NSLocale currentLocale]];
        
        // Setting components with tag, monat and jahr from the measurement:
        NSDateComponents *tagcomponents = [[NSDateComponents alloc] init];
         tagcalendar.firstWeekday = 2;
        [tagcomponents setDay:tag];
        [tagcomponents setMonth:monat];
        [tagcomponents setYear:jahr];
        
        // Getting the Date:
        NSDate *tagdatum = [tagcalendar dateFromComponents:tagcomponents];

        // Getting day:
        NSDateComponents *weekdayComponents = [tagcalendar components:(NSCalendarUnitDay | NSCalendarUnitWeekday | NSCalendarUnitMonth) fromDate:tagdatum];
        NSInteger Tag = [weekdayComponents day];
        
        // Getting weekday:
        NSUInteger TagDerWoche = [tagcalendar ordinalityOfUnit:NSCalendarUnitWeekday inUnit:NSCalendarUnitWeekOfYear forDate:tagdatum];
        
        // Getting day of year:
        NSUInteger TagDesJahres = [tagcalendar ordinalityOfUnit:NSCalendarUnitDay inUnit:NSCalendarUnitYear forDate:tagdatum];

        NSLog(@"tagdatum: %@ Tag: %d TagDerWoche: %d TagDesJahres: %d",tagdatum,Tag,TagDerWoche,TagDesJahres);
        
        // control:
        NSString* zeitzone = [[tagcalendar timeZone]abbreviation];
        NSString* lokal = [[tagcalendar locale]countryCode];
      
        NSLog(@"zeitzone: %@ lokal: %@",zeitzone, lokal); 

With NSLog, tagdatum is correct, 01.01.2020.

I wonder why, in the Variables View,

tagdatum __NSTaggedDate * 2019-12-31 23:00:00 UTC

shows the previous day.