时间格式化

DateFormats 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:JulianDayNumber(number of days since 4713 BC January1)
    G~GGG: BC/AD (EraDesignatorAbbreviated)
    GGGG:BeforeChrist/AnnoDomini

    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 TimezoneAbbreviation)
    vvvv:(General GMT TimezoneName)

    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:(FullYear)
    yy/yyy:(2DigitsYear)
    Y/YYYY:(FullYear, starting from the Sunday of the 1st week of year)
    YY/YYY:(2DigitsYear, starting from the Sunday of the 1st week of year)

    z~zzz:(Specific GMT TimezoneAbbreviation)
    zzzz:(Specific GMT TimezoneName)
    Z:+0000(RFC 822Timezone)How to Use:NSDateFormatter*dateFormatter =[[NSDateFormatter alloc] init];[dateFormatter setDateFormat:@"dd/MM/yyyy hh:mm:ss a"];NSLog(@"%@",[dateFormatter dateFromString:@"12/12/2012 12:12:12 AM"]);

更完整的需要参考

http://unicode.org/reports/tr35/tr35-10.html#Date_Format_Patterns