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