格式化日期字符串，copy一份常用参数：
a: AM/PM (上午/下午)
A: 0~86399999 (一天的第A微秒)
c/cc: 1~7 (一周的第一天, 周天为1)
ccc: Sun/Mon/Tue/Wed/Thu/Fri/Sat (星期几简写)
cccc: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday (星期几全拼)
d: 1~31 (月份的第几天, 带0)
D: 1~366 (年份的第几天,带0)
e: 1~7 (一周的第几天, 带0)
E~EEE: Sun/Mon/Tue/Wed/Thu/Fri/Sat (星期几简写)
EEEE: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday (星期几全拼)
F: 1~5 (每月的第几周, 一周的第一天为周一)
g: Julian Day Number (number of days since 4713 BC January 1) 未知
G~GGG: BC/AD (Era Designator Abbreviated) 未知
GGGG: Before Christ/Anno Domini 未知
h: 1~12 (0 padded Hour (12hr)) 带0的时, 12小时制
H: 0~23 (0 padded Hour (24hr)) 带0的时, 24小时制
k: 1~24 (0 padded Hour (24hr) 带0的时, 24小时制
K: 0~11 (0 padded Hour (12hr)) 带0的时, 12小时制
L/LL: 1~12 (0 padded Month) 第几月
LLL: Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep/Oct/Nov/Dec 月份简写
LLLL: January/February/March/April/May/June/July/August/September/October/November/December 月份全称
m: 0~59 (0 padded Minute) 分钟
M/MM: 1~12 (0 padded Month) 第几月
MMM: Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep/Oct/Nov/Dec
MMMM: January/February/March/April/May/June/July/August/September/October/November/December
q/qq: 1~4 (0 padded Quarter) 第几季度
qqq: Q1/Q2/Q3/Q4 季度简写
qqqq: 1st quarter/2nd quarter/3rd quarter/4th quarter 季度全拼
Q/QQ: 1~4 (0 padded Quarter) 同小写
QQQ: Q1/Q2/Q3/Q4 同小写
QQQQ: 1st quarter/2nd quarter/3rd quarter/4th quarter 同小写
s: 0~59 (0 padded Second) 秒数
S: (rounded Sub-Second) 未知
u: (0 padded Year) 未知
v~vvv: (General GMT Timezone Abbreviation) 常规GMT时区的编写
vvvv: (General GMT Timezone Name) 常规GMT时区的名称
w: 1~53 (0 padded Week of Year, 1st day of week = Sunday, NB: 1st week of year starts from the last Sunday of last year) 一年的第几周, 一周的开始为周日,第一周从去年的最后一个周日起算
W: 1~5 (0 padded Week of Month, 1st day of week = Sunday) 一个月的第几周
y/yyyy: (Full Year) 完整的年份
yy/yyy: (2 Digits Year) 2个数字的年份
Y/YYYY: (Full Year, starting from the Sunday of the 1st week of year) 这个年份未知干嘛用的
YY/YYY: (2 Digits Year, starting from the Sunday of the 1st week of year) 这个年份未知干嘛用的
z~zzz: (Specific GMT Timezone Abbreviation) 指定GMT时区的编写
zzzz: (Specific GMT Timezone Name) Z: +0000 ( Timezone) 指定GMT时区的名称
实例：
NSDateFormatter * dateFormatter = [[[NSDateFormatter alloc] init]autorelease];
[dateFormatter setAMSymbol:@"AM"];
[dateFormatter setPMSymbol:@"PM"];
[dateFormatter setDateFormat:@"dd/MM/yyyy hh:mmaaa"];
NSDate *date = [NSDate date];
NSString * s = [dateFormatter stringFromDate:date];
显示效果为：10/05/2010 03:49PM
下面附上几个格式：这个是从Cocoachina上来的。呵呵。
yyyy:MM:dd G 'at' HH:mm:ss zzz 1996.07.10 AD at 15:08:56 PDT
EEE, MMM d, "yy Wed,july 10, '99
h:mm a 12:08 PM
hh 'o"clock' a,zzzz 12 o'clock PM, Pacific Daylight Time
K:mm a, z 0:00 PM, PST
yyyyy,MMMM.dd GGG hh:mm aaa 01996.july.10 AD 12:08 PM
下面是得到当前的年，月，日，时，分，秒。
NSCalendar *cal = [NSCalendar currentCalendar];
unsigned int unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
NSDateComponents *dd = [cal components:unitFlags fromDate:date];
int y = [dd year];
int m = [dd month];
int d = [dd day];
int h = [dd hour];
int m = [dd minute];
int s = [dd second];