How can zman krias shma ,the start of shabbos etc. have an exact starting times if there were no clocks a thousand years ago ,if then it was am estimate why do we have an exact time now

Just like society in general, Torah constantly gets more and more exact. The tefilin found at Masada from 2000 years ago were not as square as we can make them today, but they did the best they could. Similarly, it was once enough to write that Shema was to be said when we wake up and when going to sleep, but gradually, over time, people ask more precise questions, and the answers could also become more exact.