[Libav-user] Clip audio file with specified time range!

[thanh nhan thanh nhan]

Hi all,
Does anyone know how to clip audio file with specified time range in second ( like int star_time, end_time) using ffmpeg functions.
My current status is I am running a while loop:
while (av_read_frame(formatContext, &packet) == 0)
 {
  {
   int frameFinished = 0;
   avcodec_decode_audio4(codecContext, frame, &frameFinished, &packet);
   // do somthing
  }
}
I found some fields in AVStream and AVFrame that introduce time-base, time-stamp... but i don't clearly what they mean. How can i translate those kinds of time in to real time unit( second or milisecond)
Thanks in advance!