No month appear - Problem with "cal_days_in_month()"

[ilija]

Hello,

On my calendar appear a message:
...
Warning: cal_days_in_month() [function.cal-days-in-month]: invalid date. in /home/***/public_html/www.***.com/kalendar/ac-includes/functions.inc.php on line 24"
...
Please help me to correct problem. Do I need to correct some lines in admin files or problem is with version of PHP?

Thanks, Ilija