iOS时间的创建

1. 当前时间

 NSDate * now = [NSDate date]; 

2. 根据已有时间创建新时间

NSDate * tomorrow = [NSDate dateWithTimeInterval:60*60*24 sinceDate:now];
NSDate * date1 = [NSDate dateWithTimeIntervalSince1970:0];

3. 创建某个固定时间

　　3.1 使用NSDateComponents

NSDateComponents * comp = [[NSDateComponents alloc] init];
comp.year = 2014;
comp.month = 4;
comp.day = 24;
comp.hour = 7;
comp.minute = 30;
comp.second = 30;
// 注意，这里是格林尼治，也可以选择中国，日本，美国
NSCalendar *myCal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDate *myDate = [myCal dateFromComponents:comp];
NSLog(@"%myDate = %@", myDate);
newNotification.fireDate = myDate;

　　3.2 使用NSDateFormatter

NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
formatter.timeZone = [NSTimeZone defaultTimeZone];
[formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *myDate2 = [formatter dateFromString:@"2014-04-24 7:30:30"];