Is the wp-includes/js/jquery/ui/datepicker.min.js loaded on that page? You can check that via your browser web development tools (network analysis).
It may be the case that this built-in WordPress feature is blocked by another plugin.
Hey bgermann,
got the same problem on an old site running the plugin.
I got the datepicker option selected in the admin and those files loaded:
jQuery UI Datepicker 1.11.4 (from wp-incudes)
cforms.calendar.js (from the plugin folder)
wp-includes/js/jquery/ui/datepicker.min.js
is loaded right before the
wp-content/plugins/cforms2/js/cforms.calendar.js
so the function call seems correct as far as i can see.
WP up to date (4.4).
Any clue?
Ok got it. I had two versions of jQuery running. Classic mistake of old sites.
@taghaboy Is this the reason for you, too?