iOS－Foundation框架(NSDateFormatter)

//在开发iOS程序时，有时候需要将时间格式调整成自己希望的格式，这个时候我们可以用NSDateFormatter类来处理。

//过多使用NSDateFormatter将影响程序的性能，且程序中NSDateFormatter对象的时间格式基本一致，所以使用NSDateFormatter的时候尽量使用单例模式

NSDateFormatter*dateFormatter12=[[NSDateFormatteralloc]init];

[dateFormatter12 setDateFormat:@"G yyyy-MM-dd HH:mm:ss aa"];

NSString*currentDateStr=[dateFormatter12
stringFromDate:[NSDate
date]];

NSString*date1970=[dateFormatter12stringFromDate:[NSDatedateWithTimeIntervalSince1970:0]];

NSLog(@"%@",currentDateStr);//公元
2015-12-09 16:42:17
下午

NSLog(@"%@",date1970);//公元
1970-01-01 08:00:00
上午

/*字符说明:

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)

字符格式的详细说明见
http://unicode.org/reports/tr35/tr35-10.html#Date_Format_Patterns