NSDateFormatter is the class used for formatting dates in iOS. Following table shows the symbols and corresponding usage.

Symbol Description for symbol
yyyy Year in full. eg. 2017 for 2017 and 0100 for 1st century (with prefix zeros if necessary) 
yy Year in two digits. eg. 17 for 2017
y Year in 4 digit. eg. 17 for 2017, 100 for 1st century(Year 0100)
MMMM Month in full. eg. November
MMM Month in 3 letters. eg. Nov.
MM Month in numbers. eg. 09 for Nov
M Month in numbers. eg. 9 for Nov
MMMMM Months first letter eg. N for Nov.
dd Day in numbers with prefix zero if necessary. Eg. 01 for 1.
d Day in numbers with no prefix zero. Eg. 1 for 1st January.
h Hour in 12 hour format with no prefix zero. Eg. 1 for 1 pm.
hh Hour in numbers with prefix zero if necessary. Eg. 01 pm for 1 pm.
HH Hour in 24 hour format with prefix zero if necessary. Eg. 09 for 09:00 hours.
H Hour in 24 hour format with no prefix zero. Eg. 9 for 09:00 hours
a AM or PM for 12 hour formats.
m Minutes in numbers with no prefix zero. Eg. 1 for 1 minute.
mm Minutes in numbers with prefix zeros if necessary. Eg. 01 for 1 minute.
s Seconds in numbers with no prefix zero. Eg. 1 for 1 second.
ss Seconds in numbers with prefix zeros if necessary. Eg. 01 for 1 second
zzz Timezone in 3 letters, example CST, PST, IST and so on.
zzzz Timezone in full, example Central Time Zone for CST.
Z RFC time zone format - eg. -06:00
ZZZZZ ISO 8601 time zone format - eg. -06:00
E Day in Short. eg W
EEE Day in 3 letters. eg Wed
EEEE Day in full. eg Wednesday