./program/app.js
Find and modifiy accordingly:
.
.
.
// enable general commands
this.enable_command('logout', 'mail', 'addressbook', 'settings', 'calendar', true);
.
.
.
case 'login':
var input_user = rcube_find_object('rcmloginuser');
var input_pass = rcube_find_object('rcmloginpwd');
var input_tz = rcube_find_object('rcmlogintz');
if (input_user)
input_user.onkeyup = function(e){ return rcmail.login_user_keyup(e); };
if (input_user && input_user.value=='')
input_user.focus();
else if (input_pass)
input_pass.focus();
// detect client timezone
if (input_tz)
input_tz.value = new Date().getTimezoneOffset() / -60;
this.enable_command('login', true);
break;
case 'calendar':
document.location.href("url to calendar") // or window.open for a popup
break;
... not tested, but this should do the trick!