Question

Am I missing something here? It seems like the method provided by Apple only works for UTC, regardless of the timezone default of the machine, or what you set it to.

Here’s the output I get:

Output:
2013-02-01 10:41:24.152 Scratch[17640:c07] cal=gregorian, cal.timeZone=America/Los_Angeles (PST) offset -28800
2013-02-01 10:41:24.154 Scratch[17640:c07] date_Feb1_1400PST=2013-02-01 14:00 -0800
2013-02-01 10:41:24.156 Scratch[17640:c07] date_Feb2_1200PST=2013-02-02 12:00 -0800
2013-02-01 10:41:24.157 Scratch[17640:c07] midnights between=1
2013-02-01 10:41:24.158 Scratch[17640:c07] and then...
2013-02-01 10:41:24.159 Scratch[17640:c07] date_Feb1_2000PST=2013-02-01 22:00 -0800
2013-02-01 10:41:24.161 Scratch[17640:c07] date_Feb2_1000PST=2013-02-02 10:00 -0800
2013-02-01 10:41:24.161 Scratch[17640:c07] midnights between=0

What I really want to know is “how many midnights” (i.e., how many calendar days diff) between two days for a given timezone (local or otherwise, and not necessarily UTC)

This seems like such a common and reasonably simple question that I’m surprised to see how messy and difficult to figure out.

I’m not looking for an answer that involves “mod 86400” or something filthy like that. The framework should be able to tell me this, seriously.

- (void)doDateComparisonStuff {
    NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    cal.timeZone = [NSTimeZone timeZoneWithName:@"America/Los_Angeles"];
    NSLog(@"cal=%@, cal.timeZone=%@", cal.calendarIdentifier, cal.timeZone);

    NSDate *date_Feb1_1400PST = [self dateFromStr:@"20130201 1400"];
    NSLog(@"date_Feb1_1400PST=%@", [self stringFromDate:date_Feb1_1400PST]);

    NSDate *date_Feb2_1200PST = [self dateFromStr:@"20130202 1200"];
    NSLog(@"date_Feb2_1200PST=%@", [self stringFromDate:date_Feb2_1200PST]);

    NSLog(@"midnights between=%d", [self daysWithinEraFromDate:date_Feb1_1400PST toDate:date_Feb2_1200PST usingCalendar:cal]);

    NSLog(@"and then...");

    NSDate *date_Feb1_2000PST = [self dateFromStr:@"20130201 2200"];
    NSLog(@"date_Feb1_2000PST=%@", [self stringFromDate:date_Feb1_2000PST]);

    NSDate *date_Feb2_1000PST = [self dateFromStr:@"20130202 1000"];
    NSLog(@"date_Feb2_1000PST=%@", [self stringFromDate:date_Feb2_1000PST]);

    NSLog(@"midnights between=%d", [self daysWithinEraFromDate:date_Feb1_2000PST toDate:date_Feb2_1000PST usingCalendar:cal]);
}

// based on "Listing 13" at
// https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/DatesAndTimes/Articles/dtCalendricalCalculations.html#//apple_ref/doc/uid/TP40007836-SW1
- (NSInteger)daysWithinEraFromDate:(NSDate *)startDate toDate:(NSDate *)endDate usingCalendar:(NSCalendar *)cal
{
    NSInteger startDay=[cal ordinalityOfUnit:NSDayCalendarUnit
                                       inUnit: NSEraCalendarUnit forDate:startDate];
    NSInteger endDay=[cal ordinalityOfUnit:NSDayCalendarUnit
                                     inUnit: NSEraCalendarUnit forDate:endDate];
    return endDay-startDay;
}


- (NSDate *)dateFromStr:(NSString *)dateStr {
    NSDateFormatter *df = nil;
    df = [[NSDateFormatter alloc] init];
    df.timeZone = [NSTimeZone timeZoneWithName:@"America/Los_Angeles"];
    df.dateFormat = @"yyyyMMdd HHmm";

    return [df dateFromString:dateStr];
}

- (NSString *)stringFromDate:(NSDate *)date {
    NSDateFormatter *df = nil;
    df = [[NSDateFormatter alloc] init];
    df.timeZone = [NSTimeZone timeZoneWithName:@"America/Los_Angeles"];  // native timezone here
    df.dateFormat = @"yyyy-MM-dd HH:mm Z";

    return [df stringFromDate:date];
}

Answer 1

I’ve written a small category to find the number of days between two NSDate-objects. If I understand your question correctly, that’s what you want.

NSDate+DaysDifference.h

#import <Foundation/Foundation.h>

@interface NSDate (DaysDifference)

- (NSInteger)differenceInDaysToDate:(NSDate *)otherDate;

@end

NSDate+DaysDifference.m

#import "NSDate+DaysDifference.h"

@implementation NSDate (DaysDifference)

- (NSInteger)differenceInDaysToDate:(NSDate *)otherDate {
    NSCalendar *cal = [NSCalendar autoupdatingCurrentCalendar];
    NSUInteger unit = NSDayCalendarUnit;
    NSDate *startDays, *endDays;

    [cal rangeOfUnit:unit startDate:&startDays interval:NULL forDate:self];
    [cal rangeOfUnit:unit startDate:&endDays interval:NULL forDate:otherDate];

    NSDateComponents *comp = [cal components:unit fromDate:startDays toDate:endDays options:0];
    return [comp day];
}

@end