Is this the same as #17515

It doesn't appear to be. The problem I am seeing applies to the calendar method not rendering any specified attributes. The particular 'onchange' I am trying to use is a specific example. I tested other attributes and none work. The data input is ignored.

Please try changing onchange to onChange in your code.

For JHtml helper 'calendar' method all (the new) multiple word attributes names added with the J3.7.0 new calendar are camelcase
https://github.com/joomla/joomla-cms/blob/staging/libraries/src/HTML/HTMLHelper.php#L1031-L1041

and the attribute names are not documented ?
https://docs.joomla.org/API17:JHtml::calendar
https://docs.joomla.org/JHTML/calendar

But when using the same attributes in an XML file ('calendar' form field element type)
they are lowercase which is confusing as you need to remember that --some-- attributes need to be camelcase when using JHtml instead of XML file
https://docs.joomla.org/Calendar_form_field_type

And then the layout file expects some these names to be camel case and some of the names are not only lowercase but also different name, so if you try to load the layout in custom code then you need to even account for different names ...

Just especially for onchange JHtml helper 'calendar' method
(which existed before J3.7.0 new calendar)

• the attribute name before J3.7.0 was all lowercase ??
• the attribute name with J3.7.0+ it became camelcase ??

https://github.com/joomla/joomla-cms/blob/staging/libraries/src/HTML/HTMLHelper.php#L1039

Maybe patch the JHtml helper 'calendar' method to check for both camelcase and lowercase ?

Capitalizing the C fixes the issue.