Ohanah store date/time and currency formats in the language settings. If you are using proper language in the Joomla frontend/backend, everything will be formatted properly. This includes the first day of the month. 

If you want to change date formats that are present in the language files, please see here. 

For the US date formats, please use American English (en_US) as your Joomla language. NOTE: If you switch the language in the backend, and you still don't see the new format, please log-out and log back in, that should fix it. It's a Joomla thing, don't ask :)