Category使用实例，格式化时间

////  NSDateCategory.h//  WebGameIphone////  Created by apple on 12-1-9.//  Copyright 2012年 __MyCompanyName__. All rights reserved.//#import <Foundation/Foundation.h>@interface NSDate (NSDateCategory)// 根据格式化样式得到时间字符串- (NSString *)stringWithFormat:(NSString*)fmt;// 根据字符串和格式化样式得到时间+ (NSDate *)dateFromString:(NSString*)str withFormat:(NSString*)fmt;// 改变时间字符串格式化样式+ (NSString *)dateStringReplaceFormat:(NSString*)str oldFormat:(NSString*)fmt newFormat:(NSString*)n_fmt;@end@implementation NSDate (NSDateCategory)- (NSString *)stringWithFormat:(NSString *)fmt {    static NSDateFormatter *fmtter;        if (fmtter == nil) {        fmtter = [[NSDateFormatter alloc] init];    }        if (fmt == nil || [fmt isEqualToString:@""]) {        fmt = @"HH:mm:ss";    }        [fmtter setDateFormat:fmt];        return [fmtter stringFromDate:self];}+ (NSDate *)dateFromString:(NSString *)str withFormat:(NSString *)fmt {    static NSDateFormatter *fmtter;        if (fmtter == nil) {        fmtter = [[NSDateFormatter alloc] init];    }        if (fmt == nil || [fmt isEqualToString:@""]) {        fmt = @"HH:mm:ss";    }        [fmtter setDateFormat:fmt];        return [fmtter dateFromString:str];}+ (NSString *)dateStringReplaceFormat:(NSString*)str oldFormat:(NSString*)fmt newFormat:(NSString*)n_fmt {    NSDate *n_date = [NSDate dateFromString:str withFormat:fmt];    return [n_date stringWithFormat:n_fmt];}@end