What is a mahadasha and antardasha?
Western astrology is very good at describing who you are. Vedic astrology adds a clock. While a planet's period runs, the tradition says that planet's promises in your chart come due.
The mahadashas run in this order: Sun 6 years, Moon 10, Mars 7, Rahu 18, Jupiter 16. Then Saturn 19, Mercury 17, Ketu 7 and Venus 20. Together they cover 120 years. Where you start in the cycle is set by the Moon's position at birth.
Inside each mahadasha run shorter antardashas in the same order, so the flavour of a year is the long period coloured by the short one. 'Jupiter mahadasha, Saturn antardasha' reads very differently from 'Jupiter mahadasha, Venus antardasha'.
This is why a Vedic reading can say, for example, that the window for a move runs from one month to another, where a Western one says you are the kind of person who moves. Both are true; only one is dated. In Horoskope, dated windows come with Pro.
Your current period, and the day it turns over, is computed from your birth details below. If your birth time is unknown, the dates are estimates and the reading says so. The free reading shows it when you switch it to 'I know my chart', and the birth chart calculator does the same.
The page that does this for you: Birth chart calculator.