Steps to reproduce the issue

Edit /administrator/components/com_content/models/forms/article.xml and edit the publish_up field attributes:

• remove translateformat
• add format="%Y-%m-%d %I:%M %P"
• add timeformat="12"

The final result should look something like:

		<field
			name="publish_up"
			type="calendar"
			label="COM_CONTENT_FIELD_PUBLISH_UP_LABEL"
			description="COM_CONTENT_FIELD_PUBLISH_UP_DESC"
			showtime="true"
			format="%Y-%m-%d %I:%M %P"
			timeformat="12"
			size="22"
			filter="user_utc"
		/>

Then edit any article's Start Publishing field to set the time to midnight using the calendar tool and save the changes.

Expected result

After saving, the time displaying in the text field should be 12:00 am

Actual result

The time in the text field is set to 12:00 pm

Also, the am/pm selector text is not wide enough and the option texts are hidden (see screenshot)

System information (as much as possible)

The time can be set manually in the text box to '12:00 am' and is saved as expected. But then you'll notice the hour drop down selection is empty. (See screenshot)

For the am/pm selector, it has a colspan="1" hardcoded and I believe it should be colspan="2".

Additional comments