日期与时间常用方法

##日期与时间

 1.NSDate *date = [NSDate date];
 2.NSDate *date1 = [NSDate distantFuture];//未来随机日前
 3.NSDate *date2 = [NSDate distantPast];//过去随机日期
 4.NSTimeInterval time = [date2 timeIntervalSinceDate:date1];//日期差，单位s
 5.NSDate *date3 = [NSDate dateWithTimeIntervalSinceNow:200];//当前日期加200秒
 6.NSDate *date4 = [date earlierDate:date3];//返回较早日期
//日期格式化
 7.NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
formatter.dateFormat = @"yy-MM-dd HH:mm:ss";
NSString *dateString = [formatter stringFromDate:date4];
8.//字符串转化为日期
NSDate *date5 = [formatter dateFromString:@"14-12-12 03:56:34"];