时间运算之Dates

Creating Date Objects

NSTimeInterval secondsPerDay = 24 * 60 * 60;
NSDate *tomorrow = [NSDate
            dateWithTimeIntervalSinceNow:secondsPerDay];
NSDate *yesterday = [NSDate
            dateWithTimeIntervalSinceNow:-secondsPerDay];

 

 To get new date objects with date-and-time values adjusted from existing date objects, use addTimeInterval:.

 

NSTimeInterval secondsPerDay = 24 * 60 * 60;
NSDate *today = [NSDate date];
NSDate *tomorrow, yesterday;
 
tomorrow = [today addTimeInterval:secondsPerDay];
yesterday = [today addTimeInterval:-secondsPerDay];

 

 

 

Basic Date Calculations

 

 

To compare dates, use the isEqualToDate:, compare:, laterDate:, and earlierDate: methods. These methods perform exact comparisons, which means they will detect subsecond differences between dates. You might want to compare dates with a less fine granularity. For example, you might want to consider two dates equal if they are within a minute of each other. If this is the case, use timeIntervalSinceDate: to compare the two dates or use a Gregorian date instead (NSCalendarDate in Objective-C, NSGregorianDate in Java). The following Objective-C code shows how to use timeIntervalSinceDate: to see if two dates are within one minute (60 seconds) of each other.

 

if (fabs([date2 timeIntervalSinceDate:date1]) < 60) ...