See PR #41266 & #41444

In the CalendarField, we have two format parameters:

• format: Uses by client side (javascript) to parse/format datetime value. The format is equals with the format used in PHP strftime function https://www.php.net/manual/en/function.strftime.php (which is deprecated now)
• filterformat: Uses by server side (PHP) to parser/format datetime value. The accepted format are defined here https://www.php.net/manual/en/datetime.format.php

To prevent warnings, you need to:

1. Set translateformat="true"
2. Or add filterformat attribute to your field. The value in this attribute must be equivalent to the value in format attribute. Example field:

<field
			name="created"
			type="calendar"
			label="Created Date"
			format="%Y-%m-%d"
                        filterformat="Y-m-d"
			showtime="true"
			filter="user_utc"
		/>

As the solution is there already, I'm closing this issue.