I've got Solspace Calendar installed to the best of my knowledge. I have repurposed the 'calendar_demo' files to 'calendar'. Both /calendar and /calendar_demo just show:
"Call to undefined method Calendar\Library\Carbon::setWeekStartsAt()" in DateHelper.php.
/calendar/week does the same. A path like calendar/2016/11 just give a Twig error.
I can't understand why this is happening. I'll give more details if you ask me, I don't know what else to give.