NSDateFormatter for Julian Day Number calculates with the incorrect hour in GMT.

Originator:dan
Number:rdar://11023565 Date Originated:Sat, 10 Mar 2012 00:29:31 GMT
Status:Closed Resolved:2012-05-18
Product:iPhone SDK Product Version:5.1
Classification:Other Bug Reproducible:Always
 
09-Mar-2012 05:28 PM Daniel Weeks:
Summary:

There exists for astronomical calculations a date format called the Julian Day Number (JDN, the integer count of the number of days since January 1, 4713 BCE). For example today's (2012-03-09) JDN is 2455996. The JDN is calculated from noon GMT.

The NSDateFormatter on iOS SDK and Mac OS X SDK provides a format of "g" to convert an NSDate to a Julian Day Number.

When using the NSDateFormatter to convert an NSDate to a JDN or a JDN to an NSDate the incorrect hour in GMT is used. The hour that should be used is 12:00:00 (in a 24-hour clock) and the hour of 08:00:00 is used instead.

Steps to Reproduce:

Compile the JulianDayNumber.m file with the command:

clang JulianDayNumber.m -o JulianDayNumber -std=c99 -framework Foundation -fobjc-arc

Run JulianDayNumber in a terminal. Witness the NSLog output.

Expected Results:

When converting from a string of numbers representing a JDN the resulting NSDate would be set to the day with 12:00:00 (noon) in GMT. A time of 08:00:00 would be the previous JDN.

Actual Results:

The NSDate returned is at 08:00:00 GMT when it should be at noon GMT. Any time of noon or after, until the next noon, should return a JDN of X. Any time prior to noon should return a JDN of X-1, but that only happens with times prior to 08:00:00 GMT.

Regression:


Notes:


09-Mar-2012 05:28 PM Daniel Weeks:
'JulianDayNumber.m' was successfully uploaded

Comments

Closed by Apple Engineering

Engineering has determined that this issue behaves as intended based on the following information:

This is the documented behavior of pattern character 'g' in the CLDR LDML spec, and thus in ICU. See http://www.unicode.org/reports/tr35/#Date_Field_Symbol_Table and scroll down to pattern character 'g'; it says: "Modified Julian day. This is different from the conventional Julian day number in two regards. First, it demarcates days at local zone midnight, rather than noon GMT. Second, it is a local number; that is, it depends on the local time zone. It can be thought of as a single number that encompasses all the date-related fields."

By dan at May 18, 2012, 11:42 p.m. (reply...)

JulianDayNumber.m

import

int main(int argc, char *argv[]) { @autoreleasepool { 

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"g"];
    NSLog(@"default test");
    NSLog(@"string from date: %@", [dateFormatter stringFromDate:[NSDate date]]);
    NSLog(@"date from string: %@", [dateFormatter dateFromString:[dateFormatter stringFromDate:[NSDate date]]]);

    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *components = [NSDateComponents new];
    components.timeZone = [NSTimeZone timeZoneWithName:@"GMT"];
    components.year = 2012;
    components.month = 3;
    components.day = 9;
    components.hour = 8;
    components.minute = 0;
    components.second = 0;
    NSDate *testDate = [gregorian dateFromComponents:components];
    NSLog(@"test date: %@", testDate);
    NSLog(@"2012-03-09 08:00:00 +000 test");
    NSLog(@"string from date: %@", [dateFormatter stringFromDate:testDate]);
    NSLog(@"date from string: %@", [dateFormatter dateFromString:[dateFormatter stringFromDate:testDate]]);

    components.hour = 12;
    testDate = [gregorian dateFromComponents:components];
    NSLog(@"test date: %@", testDate);
    NSLog(@"2012-03-09 12:00:00 +000 test");
    NSLog(@"string from date: %@", [dateFormatter stringFromDate:testDate]);
    NSLog(@"date from string: %@", [dateFormatter dateFromString:[dateFormatter stringFromDate:testDate]]);

    components.hour = 3;
    testDate = [gregorian dateFromComponents:components];
    NSLog(@"2012-03-09 03:00:00 +000 test");
    NSLog(@"test date: %@", testDate);
    NSLog(@"string from date: %@", [dateFormatter stringFromDate:testDate]);
    NSLog(@"date from string: %@", [dateFormatter dateFromString:[dateFormatter stringFromDate:testDate]]);
}

}

By dan at March 10, 2012, 12:30 a.m. (reply...)

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