Certain WordPress tag functions are used to display or return date and time information; the_date() and the_time() are examples of this. Some of these functions accept a parameter called a format string that allows you to determine how the date is going to be displayed. The format string is a template in which various parts of the date are combined (using "format characters") to generate a date in the format specified.
For example, the format string:
l, F j, Y
creates a date that look like this:
Friday, September 24, 2004
Here is what each format character in the string above represents:
l= Full name for day of the week (lower-case L).
F= Full name for the month.
j= The day of the month.
S= The English suffix for the day of the month (e.g st, nd or th in the 1st, 2nd or 15th.)
Y= The year in 4 digits. (lower-case y gives the year's last 2 digits)
WordPress is written in the programming language PHP. Therefore, the date formatting functions in WordPress use PHP's built-in date formatting functions. You can use the table of date format characters on the PHP website as a reference for building date format strings for use in WordPress.