Calendar only works on index page

I set up my blog so that the WP index is in my main directory… all of my other content pages are in my main directory as well. My php function for my calendar is on my menu, which I include via php. It only works on my index tho… I don’t understand why. Here is the error that I am getting:
Fatal error: Call to undefined function: get_calendar() in /home/bluetst/public_html/skins/3/menu.php on line 119