iphone - NSDate减去一个月

我想要两个NSInteger年，一个月再回一个月。例如现在是 2011-01 我怎么能获得 2010-12 。加是一回事，但我想减去。

NSDate *date = [NSDate date];
NSDateComponents *dateComponents = [calendar components:unitFlags fromDate:date];
NSInteger year = [dateComponents year];
NSInteger month = [dateComponents month];
[calendar release];
NSString * pdf_name = [NSString stringWithFormat:@"file_%d_%02d.pdf", year, month];

问候内

编辑:我的解决方案

    NSCalendar *calendar= [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
 NSCalendarUnit unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;

 NSDate *date = [NSDate date];
 NSDateComponents *dateComponents = [calendar components:unitFlags fromDate:date];
 NSInteger year = [dateComponents year];
 NSInteger month = [dateComponents month];

 BOOL substractOneMonth = YES;
 if (substractOneMonth) {
  if (month==1) {
   year--;
   month = 12;
  }else {
   month--;
  }
 }
 [calendar release];

 NSString * pdf_name = [NSString stringWithFormat:@"file_%d_%02d.pdf", year, month];

最佳答案

此示例代码来自iPhone的“日期和时间编程指南”。

NSDate *today = [[NSDate alloc] init];
NSLog(@"%@", today);
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *offsetComponents = [[NSDateComponents alloc] init];
[offsetComponents setMonth:-1]; // note that I'm setting it to -1
NSDate *endOfWorldWar3 = [gregorian dateByAddingComponents:offsetComponents toDate:today options:0];
NSLog(@"%@", endOfWorldWar3);

我希望这有帮助!