In the article comparing Vedic vs Western astrology, I made a list of techniques used to predict life events. As per my research, the planetary time periods calculated using "Vimshottari" Dasha system is the most accurate method of predicting life events. All the classics written on Vedic astrology has written extensively about this method and I've personally found this method to be very accurate.

So, what’s the science behind it?

It’s based on the position of the Moon. Moon travels 13 degrees 20 minutes in a day, and the path it covers is called a Lunar Mansion or a Nakshatra. It takes Moon ~27 days to travel across the entire zodiac (360 degrees) so there are 27 Nakshatras (click to read more), which play a pivotal role in Vedic astrology.

Each Nakshtra is ruled by a planet for a specific number of years (time period) based on the Vimshottari Dasha system. So depending on the degree of Moon in your birth chart, you go through the time period of the ruler of the Nakshatra - and that ruler (planet) affects your life the most. Take a look at this table.

Let's use an example to study this table

Let's say you were born 11th April 1990 at 18 30 in London, then as per your birth chart your Moon’s degree is 15 degrees Libra, so what that means is that Moon has travelled 6 signs (Aries to Virgo) of 30 degrees each, and another 15 degrees of Libra taking the Longitude of Moon, in your birth chart, to 195 degrees (6x30 + 15).

So look at the table shown above, you could see you were born in the Nakshatra of Swati that is ruled by Rahu. But Moon has already traveled 8 degrees and 20 minutes (195 - 186.33) of the 13 degrees 20 minutes of the Nakshatra so you’ll go through the remaining years as per this formula.

Time period of Rahu left = (13.33 - 8.33)/ 13.33 * 18 years = 6 years and 9 months

After you have finished the remaining time period of Rahu, you'll enter the time period of the next ruling planet in the sequence (refer table above), which in this case is Jupiter for 16 years, followed by Saturn for 19 years and so on.

MORE CALCULATIONS*

*you could move on to the next section if you are more interested in the application of this method.
The main time period is further broken down to a sub-time period and a sub-sub time period. For the sake of simplicity, I call them time period#1, #2, and #3. So how are they broken down? if you are starting a time period#1 of a new planet, then time period#2, and #3 are also of the same planet. Now time period#3 (ruling planet) first changes as per the sequence in the table above, and once it completes the cycle, only then does time period#2 changes, and after its cycle is over that is when time period#1 changes. Let's say you starting a time period of Sun, so time period#1,#2, and #3 are all Sun. The next time period will be Sun-Sun-Moon, followed by Sun-Sun-Mars....all the way till Sun-Sun-Venus, and then it becomes Sun-Moon-Moon, and the same cycle follows, until time period #1 changes. Another interesting question is how to calculate the duration of these time periods. Let's take Sun-Sun-Sun as an example, as per the table the duration of Sun's time period is 6 years, and as per vimshottari dasha system the life of a person is 120 years, for the purpose of calculations. So, the duration of Sun-Sun will be = 6 x 6/120 = 109 days, and Sun-Sun-Sun = 109* 6/120 = 5 days.
You don't need to worry about the calculations they've already been done on this website.

Here's how to use it

The life of every human being is taken as 120 years and it is divided into different phases controlled by different planets. You could find the planets controlling your life right now by navigating to the table on farfaraway that says "planetary time periods" (as shown above). In this table, you'll find 4 columns: 1. Time period #1, which is the main time period. 2. Time period #2, which is the sub time period. 3. Time period #3, which is the sub-sub time period. 4. The end date i.e. that the date till which this phase will last. Now, the planets in time periods #1 & #2 are responsible for creating the circumstances for a life event to take place whereas the planet in time period #3 is responsible for executing that life event at that time.

Let's take an example. If the planet in time period #1 is Jupiter, the planet in time phase #2 is the lord of the 7th house, and the planet in time phase #3 is sitting in the 11th house then this time period would create a very high possibility of marriage. Since Jupiter (blessings) is joining the planet controlling (lord) the 7th house (marriage) and the planet sitting in the 11th house (desires). I hope you are able to see this connection. Similarly, if at a certain time the planets controlling your life are having a strong connection with the career related houses (the 10th & the 6th) then a career-related event is likely to happen at that time.

final words

Check how planets are linked in your birth chart e.g. if you are trying to predict your time of marriage then see which planets are ruling the 7th house in your birth chart, which planets are sitting in it, and which planets are aspecting it. Now do the same for the 11th house. Now check when in the future are you going to go through a combination of planets which influence the marriage house (7th) and the house of desires (11th). The phase of those planets would be your time of marriage :)