字符串转成NSDate类型，计算与当前时间的相差，年数，天数，时分秒

NSString *dateStr=@"2013-08-13 20:28:40";//传入时间
//将传入时间转化成需要的格式
NSDateFormatter *format=[[NSDateFormatter alloc] init];
[format setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *fromdate=[format dateFromString:dateStr];
NSTimeZone *fromzone = [NSTimeZone systemTimeZone];
NSInteger frominterval = [fromzone secondsFromGMTForDate: fromdate];
NSDate *fromDate = [fromdate  dateByAddingTimeInterval: frominterval];
NSLog(@"fromdate=%@",fromDate);
[format release];
//获取当前时间
NSDate *date = [NSDate date];
NSTimeZone *zone = [NSTimeZone systemTimeZone];
NSInteger interval = [zone secondsFromGMTForDate: date];
NSDate *localeDate = [date  dateByAddingTimeInterval: interval];
NSLog(@"enddate=%@",localeDate);

double intervalTime = [fromDate timeIntervalSinceReferenceDate] - [localeDate timeIntervalSinceReferenceDate];

long lTime = (long)intervalTime;
NSInteger iSeconds = lTime % 60;
NSInteger iMinutes = (lTime / 60) % 60;
NSInteger iHours = (lTime / 3600)$;
NSInteger iDays = lTime/60/60/24;
NSInteger iMonth = lTime/60/60/24/12;
NSInteger iYears = lTime/60/60/24/384;

NSLog(@"相差M年d月 或者 d日d时d分d秒", iYears,iMonth,iDays,iHours,iMinutes,iSeconds);
//该代码片段来自于: http://www.sharejs.com/codes/objectc/5099