Last Updated: February 25, 2016
· cybersamx
Portrait sam

Quick Guide to Objective-C DateFormatting

The class NSDateFormatter allows us to define the date format of the textual representation of the date and time in iOS/Cocoa.

Below is a summary of the most commonly used specifiers used in the date format string (keep in mind that they are case sensitive):

  • y = year
  • Q = quarter
  • M = month
  • w = week of year
  • W = week of month
  • d = day of the month
  • D = day of year
  • E = day of week
  • a = period (AM or PM)
  • h = hour (1-12)
  • H = hour (0-23)
  • m = minute
  • s = second

In general, the number of characters in a specifier determine the size of date field. Let’s use an example to illustrate date formatting.

eg. Input date = 2011-05-01 Sunday

1-character = 1-digit/character number or word (if number/word can’t be 1 character long then abbreviation or fullname is displayed).

[dateFormatter setDateFormat:@"E, d M y"];  // Output: Sun, 1 5 2011

2-character = 2-digit/character number or word (if number/word can’t be 2 character long then abbreviation is displayed).

[dateFormatter setDateFormat:@"EE, dd MM yy"];  // Output: Sun, 01 05 11

3-character = 3-digit/character number or word, or abbreviation (generally).

[dateFormatter setDateFormat:@"EEE, ddd MMM yyy"];  // Output: Sun, 001 May 2011

4-character = full name (generally).

[dateFormatter setDateFormat:@"EEEE, dddd MMMM yyyy"];  // Output: Sunday, 0001 May 2011

Here’s the weird part though, if you specify 5 E’s, you get an rather unexpected output. You would think that the output date field would be longer than 1 character:

[dateFormatter setDateFormat:@"EEEEE, ddddd MMMMM yyyyy"];  // Output: S, 00001 M 2011

For date formatting, the following reference table has been very useful:

Date Field Symbol Table - UTS #35 Unicode Locale Data Markup Language

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As side note, to anyone who doesn't know, date formatters should be reused as much as possible. Also changing the date format is almost as costly as creating a new date formatter so it is often wise to create multiple instances with different date formats.

over 1 year ago ·
Portrait sam

That's a good point. I usually instantiate NSDateFormatter as a static variable especially if I know that the date format is static and pre-defined.

NSString *getDateStringFromDate(NSDate *date) {
  static NSDateFormatter *dateFormatter = nil;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
     dateFormatter = [[NSDateFormatter alloc] init];
     // Output: 2011-05-01 13:15:08
    dateFormatter.dateFormat = @"yyyy-MM-dd HH:mm:ss";

   return [dateFormatter stringFromDate:date];
over 1 year ago ·
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