Nearly every date in Drupal's display can be modified by change the date format. Here we show how to adjust the date in the "Submitted by" line but also warn that doing so can have unintended consequences.
So this format is set in our date and time format settings, let's go there. I'm going to type in "date" and I'll click on "Date and time formats. " By default, the submitted by line and a number of other places on this site use this default medium date format.
We can adjust this. I'm going to click "Edit" and what you see here is the name of the format plus the format string. This is a special code that lines up with particular date pieces and the format of those date pieces.
There's a link here to the php manual. I'm going to open it up in a new tab and let's take a look at what this capital D means. So I'm going to go to the date and I'll scroll down and you can see the character here that you can use, these are grouped so you can see that these are all under day and there's a week and month grouping below it.