Hello,

Datetime fields in frontend is using Joomla core calendar form field to generate the datetime picker.

The issue you have shows that your site template does not provide needed styling for calendar form types (check publishing calendar picker for Joomla articles edition in frontend and you may have the same issue).

So better to contact your site template provider to fix this (or to use a Joomla 3 compatible site template).

Hope this helps?

Best regards,
Cyril

Note: if you have an option in your site template to enable Bootstrap, so you should turn this option "on" so that the button could be rendered...