Check out the Swazz Calendar. Small file size, easy to integrate and actually works.
One caveat: by default, it sets the date the European way (dd/mm/yyyy). To change to the American format, find the following line in the prepcalendar function:
calvalarr[d]=”+(d-cd)+’/'+(cm-(-1))+’/'+cy;
and change it to:
calvalarr[d]=”+(cm-(-1))+’/'+(d-cd)+’/'+cy;
Then find these lines in function lcs:
ccm=curdtarr[1]-1;
ccy=curdtarr[2];
prepcalendar(curdtarr[0],curdtarr[1]-1,curdtarr[2]);
Change them to:
ccm=curdtarr[0]-1;
ccy=curdtarr[2];
prepcalendar(curdtarr[1],curdtarr[0]-1,curdtarr[2]);
Voilà. (That means “There you go” in English. 
)