I am not familiar with mikroC compiler. But i can tell u something..
If you are going to just program the existing timer to count 30 seconds, you can use the timer instructions to make it run for 30 seconds, in which you would get the output in hexadecimal values which you can convert to BCD and then use IC7447 to convert it to 7 segment format..
If you are going to design a custom timer, you need to get the clock signal of the microcontroller and just make a counter to count the no. of clock signals.Since the clock time period of the microcontroller is known you must calculate how much cycles does it need to make 1 second. At this value make the counter reset and use another counter to keep track of the seconds.Now You just need to get the output of the second counter to the display.Make the counter to be reseted after 30 seconds...