iOS development

Cache Formatters for Efficiency
Creating a date formatter is not a cheap operation. If you are likely to use a formatter frequently, it is typically more efficient to cache a single instance than to create and dispose of multiple instances. One approach is to use a static variable.

If you cache date formatters (or any other objects that depend on the user’s current locale), you should subscribe to the NSCurrentLocaleDidChangeNotification notification and update your cached objects when the current locale changes. The code in Listing 3 defines sUserVisibleDateFormatter outside of the method so that other code, not shown, can update it as necessary. In contrast, sRFC3339DateFormatter is defined inside the method because, by design, it is not dependent on the user’s locale settings.

Reference: Date Formatters

NSDateFormatter formatting

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)

References:
1. NSDateFormatter formatting
2. Date Formatters
3. Date Format Patterns