How do I get the day of the week in Objective-C?

dayOfTheWeek.m

- (void)viewDidLoad
{
    NSDate *currentDate = [NSDate date];
    NSLog(@"Current Date: %@", currentDate);
    
    // Outputs current day of week as a string in locale dependent on current regional settings.
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"EEEE"];
    NSLog(@"The day of the week: %@", [dateFormatter stringFromDate:[NSDate date]]);
    
    // To get just a week day number you must use NSCalendar class
    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *comps = [gregorian components:NSWeekdayCalendarUnit fromDate:[NSDate date]];
    int weekday = [comps weekday];
    NSLog(@"The week day number: %d", weekday);
}

via: here

NSDateFormatter 格式说明：

• G: 公元时代，例如AD公元
• yy: 年的后2位
• yyyy: 完整年
• MM: 月，显示为1-12
• MMM: 月，显示为英文月份简写,如 Jan
• MMMM: 月，显示为英文月份全称，如 Janualy
• dd: 日，2位数表示，如02
• d: 日，1-2位显示，如 2
• EEE: 简写星期几，如Sun
• EEEE: 全写星期几，如Sunday
• aa: 上下午，AM/PM
• H: 时，24小时制，0-23
• K：时，12小时制，0-11
• m: 分，1-2位
• mm: 分，2位
• s: 秒，1-2位
• ss: 秒，2位
• S: 毫秒

常用日期结构：

• yyyy-MM-dd HH:mm:ss.SSS
• yyyy-MM-dd HH:mm:ss
• yyyy-MM-dd
• MM dd yyyy

via: click

要想将 2014-06-04 00:00:00 的字符串中抽取 06-04 出来：

- (NSString *)translateDateString:(NSString *)dateString FromFormat:(NSString *)oldFormate toFormat:(NSString *)updateFormat{
    NSDateFormatter *dateFormater = [[NSDateFormatter alloc] init];
    [dateFormater setDateFormat:oldFormate];
    NSDate *timeDate = [dateFormater dateFromString:dateString];
    [dateFormater setDateFormat:updateFormat];
    return [dateFormater stringFromDate:timeDate];
}

NSString *string =  [self translateDateString:dateString FromFormat:@"yyyy-MM-dd HH:mm:ss" toFormat:@"MM-dd"];

Convert NSDate in to relative format as “Today”,“Yesterday”:

#define SECOND  (1)
#define MINUTE  (60 * SECOND)
#define HOUR    (60 * MINUTE)
#define DAY     (24 * HOUR)
#define MONTH   (30 * DAY)
#define YEAR    (12 * MONTH)

- (NSString *)timeAgo:(NSdate *)date
{
    NSTimeInterval delta = [[NSDate date] timeIntervalSinceDate:date];

    if (delta < 1 * MINUTE)
    {
        return @"刚刚";
    }
    else if (delta < 2 * MINUTE)
    {
        return @"1分钟前";
    }
    else if (delta < 45 * MINUTE)
    {
        int minutes = floor((double)delta/MINUTE);
        return [NSString stringWithFormat:@"%d分钟前", minutes];
    }
    else if (delta < 90 * MINUTE)
    {
        return @"1小时前";
    }
    else if (delta < 24 * HOUR)
    {
        int hours = floor((double)delta/HOUR);
        return [NSString stringWithFormat:@"%d小时前", hours];
    }
    else if (delta < 48 * HOUR)
    {
        return @"昨天";
    }
    else if (delta < 30 * DAY)
    {
        int days = floor((double)delta/DAY);
        return [NSString stringWithFormat:@"%d天前", days];
    }
    else if (delta < 12 * MONTH)
    {
        int months = floor((double)delta/MONTH);
        return months <= 1 ? @"1个月前" : [NSString stringWithFormat:@"%d个月前", months];
    }

    int years = floor((double)delta/MONTH/12.0);
    return years <= 1 ? @"1年前" : [NSString stringWithFormat:@"%d年前", years];
}

via NSDate+BeeExtension.h

How can I calculate the difference between two dates?

NSDate *date1 = [NSDate dateWithString:@"2010-01-01 00:00:00 +0000"];
NSDate *date2 = [NSDate dateWithString:@"2010-02-03 00:00:00 +0000"];

NSTimeInterval secondsBetween = [date2 timeIntervalSinceDate:date1];

int numberOfDays = secondsBetween / 86400;

NSLog(@"There are %d days in between the two dates.", numberOfDays);

via How can I calculate the difference between two dates?

Get local current time string:

NSDate *today = [NSDate date];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setTimeZone:[NSTimeZone localTimeZone]];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSString *currentTime = [dateFormatter stringFromDate:today];

DateFormatter:

yyyy-MM-dd'T'HH:mm:ss.ZZZZ
2015-04-27T03:02:52+00:00