Why does NSDateFormatter return nil date for these

问题:

Try running this in iOS6 (haven't tested pre iOS6):

NSDateFormatter *julianDayDateFormatter = nil;
julianDayDateFormatter = [[NSDateFormatter alloc] init];
[julianDayDateFormatter setDateFormat:@"g"];

for (NSString *timeZone in [NSTimeZone knownTimeZoneNames]) {
    julianDayDateFormatter.timeZone = [NSTimeZone timeZoneWithName: timeZone];
    NSDate *date = [julianDayDateFormatter dateFromString:[NSString stringWithFormat:@"%d", 2475213]];
    if (date == nil)
        NSLog(@"timeZone = %@", timeZone);
}

and you get the following output:

America/Bahia
America/Campo_Grande
America/Cuiaba
America/Sao_Paulo

Can anyone explain why these four time zones behave like this with NSDateFormatter set to julian day numbers? All other time zones makes NSDateFormatter return actual NSDates.

回答1:

I have a suspicion. Only a suspicion, but a pretty strong one.

That value represents October 19th 2064. The Brazilian time zones observe daylight saving time starting at local midnight - that's when their clocks go forward, so midnight itself doesn't exist. October 19th is one of those transitions.

Here's some sample code using Noda Time, my .NET date/time API. It checks whether the start of the day in every time zone it knows about is actually midnight:

using System;
using NodaTime;

class Test
{
    static void Main()
    {
        var localDate = new LocalDate(2064, 10, 19);
        var provider = DateTimeZoneProviders.Tzdb;
        foreach (var id in provider.Ids)
        {
            var zone = provider[id];
            var startOfDay = zone.AtStartOfDay(localDate).LocalDateTime.TimeOfDay;
            if (startOfDay != LocalTime.Midnight)
            {
                Console.WriteLine(id);
            }
        }
    }
}

That produces a very similar list:

America/Bahia
America/Campo_Grande
America/Cuiaba
America/Sao_Paulo
Brazil/East

I suspect Brazil/East may be an alias for America/Sao_Paolo, which is why it's not on your list.

Anyway, to get back to your Julian day issue - I suspect the formatter always wants to return an NSDate * which is at the local midnight. That doesn't exist for October 19th 2064 in those time zones... hence it returns nil. Personally I'd suggest it should return the 1am value instead, but hey...

回答2:

Credits to Jon Skeet for putting me on the right track. However, I just want to clarify his answer in an iOS context.

When you ask NSDateFormatter to convert a julian day number into an NSDate, you can only specify whole numbers (usually you can specify a decimal part for the hours/minutes/secs of the day) in the string to be parsed. Because Apple demarcates julian days at midnight (as opposed to noon in astronomy, read more here: http://www.unicode.org/reports/tr35/#Date_Field_Symbol_Table) and some midnights simply doesn't exists (thanks for pointing that out @JonSkeet) NSDateFormatter identifies that that particular point in time doesn't exist in that time zone and returns nil.

For the record, iOS5 does not behave like this and I agree with Jon Skeet, that NSDateFormatter should return an NSDate adjusted for DST instead of nil, as that particular julian day in fact exists! I filed a bug with Apple.