# Extra Clang Tools 21.1.6 documentation

## clang-tidy - objc-nsdate-formatter

# objc-nsdate-formatter

When `NSDateFormatter` is used to convert an `NSDate` type to a `String` type, the user can specify a custom format string. Certain format specifiers are undesirable despite being legal. See http://www.unicode.org/reports/tr35/tr35-dates.html#Date_Format_Patterns for all legal date patterns.

This checker reports as warnings the following string patterns in a date format specifier:

1. yyyy + ww : Calendar year specified with week of a week year (unless YYYY is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: yyyy-ww;
     Output string: 2014-01 (Wrong because it’s not the first week of 2014)
   - Example 2: Input Date: 29 December 2014 ; Format String: dd-MM-yyyy (ww-YYYY);
     Output string: 29-12-2014 (01-2015) (This is correct)

2. F without ee/EE : Numeric day of week in a month without actual day.

   - Example: Input Date: 29 December 2014 ; Format String: F-MM;
     Output string: 5-12 (Wrong because it reads as 5th ___ of Dec in English)

3. F without MM : Numeric day of week in a month without month.

   - Example: Input Date: 29 December 2014 ; Format String: F-EE
     Output string: 5-Mon (Wrong because it reads as 5th Mon of ___ in English)

4. WW without MM : Week of the month without the month.

   - Example: Input Date: 29 December 2014 ; Format String: WW-yyyy
     Output string: 05-2014 (Wrong because it reads as 5th Week of ___ in English)

5. YYYY + QQ : Week year specified with quarter of normal year (unless yyyy is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: YYYY-QQ
     Output string: 2015-04 (Wrong because it’s not the 4th quarter of 2015)
   - Example 2: Input Date: 29 December 2014 ; Format String: ww-YYYY (QQ-yyyy)
     Output string: 01-2015 (04-2014) (This is correct)

6. YYYY + MM : Week year specified with Month of a calendar year (unless yyyy is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: YYYY-MM
     Output string: 2015-12 (Wrong because it’s not the 12th month of 2015)
   - Example 2: Input Date: 29 December 2014 ; Format String: ww-YYYY (MM-yyyy)
     Output string: 01-2015 (12-2014) (This is correct)

7. YYYY + DD : Week year with day of a calendar year (unless yyyy is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: YYYY-DD
     Output string: 2015-363 (Wrong because it’s not the 363rd day of 2015)
   - Example 2: Input Date: 29 December 2014 ; Format String: ww-YYYY (DD-yyyy)
     Output string: 01-2015 (363-2014) (This is correct)

8. YYYY + WW : Week year with week of a calendar year (unless yyyy is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: YYYY-WW
     Output string: 2015-05 (Wrong because it’s not the 5th week of 2015)
   - Example 2: Input Date: 29 December 2014 ; Format String: ww-YYYY (WW-MM-yyyy)
     Output string: 01-2015 (05-12-2014) (This is correct)

9. YYYY + F : Week year with day of week in a calendar month (unless yyyy is also specified).

   - Example 1: Input Date: 29 December 2014 ; Format String: YYYY-ww-F-EE
     Output string: 2015-01-5-Mon (Wrong because it’s not the 5th Monday of January in 2015)
   - Example 2: Input Date: 29 December 2014 ; Format String: ww-YYYY (F-EE-MM-yyyy)
     Output string: 01-2015 (5-Mon-12-2014) (This is correct)