have to send sms for every 30 mins in php

Hi

I have a logical problem , I have to send an sms to registered user for every 30 mins if the connection is offline. my question here is, it will automatically excute the program for every 2 mins.. if the connection is offline then it will send an sms first time then after it will check the time 30 mins, after that for every 30 mins it will send an sms again. can any one change my code to satisfy these conditions

please check my code

 if ( $plant['modbus_error']==11 && $plant['extra']['check']=="on" )
    {
   
          
          
            $count=0;
          
           $message="Connection Alerts: ";
           $message.=$plant['description'];
           $message.="@";
           $message.=time();
           $message.="@";
           $message.="Logger is offline";
           if($count<1)
           {
           $time_limit=$plant['extra']['time_limit'];
            $time_limit=$time_limit*60;
           $time_calc=time()+$time_limit;
           }
          
           echo $time_calc;
          
           if($time_calc<time() || $count==0)
           {
            
           for ($i=0; $i<count($plant['extra']['contact']['sms']);$i++)
             {
            
                     
             $error_co= techstudio_sms_send (  $plant['extra']['contact']['sms'][$i], $message );
              
              
             }
           }
    }