Objective-c NSDate 取时间，日期，星期，毫秒等

NSDate

//得到当前的日期
NSDate *date = [NSDate date];
NSLog(@"date:%@",date);

//得到(24 * 60 * 60)即24小时之前的日期，dateWithTimeIntervalSinceNow:
NSDate *yesterday = [NSDate dateWithTimeIntervalSinceNow: -(24 * 60 * 60)];
NSLog(@"yesterday:%@",yesterday);

NSDateFormatter *formatter =[[[NSDateFormatter alloc] init] autorelease];
NSDate *date = [NSDate date];
[formatter setTimeStyle:NSDateFormatterMediumStyle];
NSCalendar *calendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *comps = [[[NSDateComponents alloc] init] autorelease];
NSInteger unitFlags = NSYearCalendarUnit |
NSMonthCalendarUnit |
NSDayCalendarUnit |
NSWeekdayCalendarUnit |
NSHourCalendarUnit |
NSMinuteCalendarUnit |
NSSecondCalendarUnit;
//int week=0;
comps = [calendar components:unitFlags fromDate:date];
int week = [comps weekday];
int year=[comps year];
int month = [comps month];
int day = [comps day];
//[formatter setDateStyle:NSDateFormatterMediumStyle];
//This sets the label with the updated time.
int hour = [comps hour];
int min = [comps minute];
int sec = [comps second];
NSLog(@"week%d",week);
NSLog(@"year%d",year);
NSLog(@"month%d",month);
NSLog(@"day%d",day);
NSLog(@"hour%d",hour);
NSLog(@"min%d",min);
NSLog(@"sec%d",sec);

//得到毫秒
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateStyle:NSDateFormatterMediumStyle];
[dateFormatter setTimeStyle:NSDateFormatterShortStyle];
//[dateFormatter setDateFormat:@"hh:mm:ss"]
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss.SSS"];
NSLog(@"Date%@", [dateFormatter stringFromDate:[NSDate date]]);
[dateFormatter release];

iPhone NSDate 时间间隔

NSDate *beginTime = [NSDate date];
....
NSTimeInterval loadDuration = [beginTime timeIntervalSinceNow];

关于NSTimeInterval的使用

NSTimeInterval time = [[NSDate date] timeIntervalSince1970]; 可以这样定义
NSLog(@"当前时间为%f", time); 打印出从1970年开始到现在的秒数.

可见time可以赋值给float类型
所以可以这样写：
float a ＝ time；

这样就可以很容易的到两个时间点中间相差多少秒。