Core Data under Japanese Calendar incorrectly comparing dates

Question

So I seem to be getting burned by the iPhone's calendar support again. I've come across an issue with the way core data compares dates. Core data appears to be storing dates using the Gregorian calendar as they come out in a 20XX-MM-dd format. But when I build an NSPredicate this way

[nsRequest setPredicate:[NSPredicate predicateWithFormat:@"publish <= %@ AND expires >= %@", [NSDate date], [NSDate date]]]

if the user's device isn't set to a Gregorian calendar then no items will appear, as core data doesn't seem to localizing it's NSDates before comparing against the input dates.

So it would appear that if the date were today '2011-05-31' in Japanese calendar mode core data is using '0023-05-31' since it's the 23rd year of the current emperor, placing the input date about 2000 years before what it's being compared against.

I've found I can work around this like so

NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:[[NSLocale preferredLanguages] objectAtIndex:0]] autorelease]];
[dateFormat setDateFormat:@"yyyy-MM-dd HH:mm:ss ZZZ"];
NSString *todayGregorian = [[dateFormat stringFromDate:[NSDate date]] retain];
[dateFormat release];

NSDateFormatter *df = [[NSDateFormatter alloc] init];
[df setDateFormat:@"yyyy-MM-dd HH:mm:ss ZZZ"];
[nsRequest setPredicate:[NSPredicate predicateWithFormat:@"publish <= %@ AND expires >= %@", [df dateFromString:todayGregorian], [df dateFromString:todayGregorian]]];

but I'm hoping that maybe I'm being an idiot and there's a better way. Any suggestions?

Please note:

Asking my Japanese users to change the date format they use isn't an option.
Also, I still need to store the core data dates in a Gregorian format, as these users do sometimes change their calendars back and forth.

Real problem My solution above is wrong. TechZen's answer kicked my head in to gear and I wish I could up vote him 100 times, I ended up solving the issue.

My problem is data going in to Core Data is being sent by the server in GMT and the formmater parsing that data might not necessarily be in GMT so I need to adjust the time going in to Core Data correctly.

TechZen TechZen · Accepted Answer · 2011-05-31T15:42:53

You really have no choice but to translate from NSDate to various calendars, especially if your users employ multiple calendars.

Calendar objects are not dates themselves but rather specialized formatters. All dates are actually stored in the GMT standard format regardless of the calendar they originated with. Then they are translated/formatted to the appropriate calendar as needed. We don't see this immediately in apps targeted towards Westerners because contemporary Western calendars are based on the GMT so no translation appears needed.

Date and time programming is deceptively complex. We don't intuitive feel it is complex because so much of the grunt work is usually handled for us. However, when you start doing something unusual, the complexity because readily apparent.

As a rule of thumb, the time to worry with data and time programming is not when the code seems to complex but when it seems to simple.

Core Data under Japanese Calendar incorrectly comparing dates

1 Answers