Natal horoscope indicate what may happen and when it will happen is determined by Dasha. Default dasha considered is vimshotari  calculated based on star of  moon. It goes in predefined way ketu venus sun moon mars rahu Jupiter Saturn and mercury 7 20 6 10 7 18 16 19 and 17 years respectively for each planets. Good and bad affect are predicted based on planets or dasha lords natural and functional portfolios.  Apart from vimshotari dasha there are other dasha also runs   along with vimshotary. Its astrologers expertise which one he analyzes and predicts. Other important dasha are kaal chakra dasha char dasha astotary dasha.  Few astrologer also calculate dasha from other reference special points in horoscope.