cal_days_in_month() is a function name that is part of PHP from version 4.1.0 – I am afraid that is a problem with your web host
They are running either a really old version of PHP or have not enabled the calendar functions. (They needed to have complied php with –enable-calendar)
Please get in touch with your host’s technical support and ask why the PHP function cal_days_in_month is not working!