http://blog.seymourdev.com/?p=754
NSDateFormatter 支持的格式化字符串
a: AM/PM
A: 0~86399999 (Millisecond of Day)
c/cc: 1~7 (Day of Week)
ccc: Sun/Mon/Tue/Wed/Thu/Fri/Sat
cccc: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday
d: 1~31 (0 padded Day of Month)
D: 1~366 (0 padded Day of Year)
e: 1~7 (0 padded Day of Week)
E~EEE: Sun/Mon/Tue/Wed/Thu/Fri/Sat
EEEE: Sunday/Monday/Tuesday/Wednesday/Thursday/Friday/Saturday
F: 1~5 (0 padded Week of Month, first day of week = Monday)
g: Julian Day Number (number of days since 4713 BC January 1)
G~GGG: BC/AD (Era Designator Abbreviated)
GGGG: Before Christ/Anno Domini
h: 1~12 (0 padded Hour (12hr))
H: 0~23 (0 padded Hour (24hr))
k: 1~24 (0 padded Hour (24hr)
K: 0~11 (0 padded Hour (12hr))
L/LL: 1~12 (0 padded Month)
LLL: Jan/Feb/Mar/Apr/May/Jun/Jul/Aug/Sep/Oct/Nov/Dec
LLLL: January/February/March/April/May/June/July/August/September/October/November/December
m: 0~59 (0 padded Minute)
M/MM: 1~12 (0 padded Month)
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)
qqq: Q1/Q2/Q3/Q4
qqqq: 1st quarter/2nd quarter/3rd quarter/4th quarter
Q/QQ: 1~4 (0 padded Quarter)
QQQ: Q1/Q2/Q3/Q4
QQQQ: 1st quarter/2nd quarter/3rd quarter/4th quarter
s: 0~59 (0 padded Second)
S: (rounded Sub-Second)
u: (0 padded Year)
v~vvv: (General GMT Timezone Abbreviation)
vvvv: (General GMT Timezone Name)
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)
W: 1~5 (0 padded Week of Month, 1st day of week = Sunday)
y/yyyy: (Full Year)
yy/yyy: (2 Digits Year)
Y/YYYY: (Full Year, starting from the Sunday of the 1st week of year)
YY/YYY: (2 Digits Year, starting from the Sunday of the 1st week of year)
z~zzz: (Specific GMT Timezone Abbreviation)
zzzz: (Specific GMT Timezone Name)
Z: +0000 (RFC 822 Timezone)
