How do you find a planet's perihelion date?
You can get an approximate date by adding a multiple of the planet's period to a past perihelion data. To get an accurate date requires a lot of computation.
Perihelion is actual a moment in time when the planet is at its closest point to its sun.
The time between one perihelion and the next is called the anomalistic period. If the planet follows a Kepler elliptical orbit each perihelion will occur one anomalistic period after the previous one.
Planets however do not follow true Kepler elliptical orbits due to precession and the gravitational effects of other planets.
Earth's perihelion occurs around the 3rd of January. The actual time of perihelion can vary from 2nd - 5th January dependent on the positions of the other planets, particularly Venus and Jupiter.
NASA's Jet Propulsion Laboratory provide data files which allow the positions of the Sun, Moon and other planets to be calculated with great accuracy for any time from centuries ago to centuries in the future. The DE430 data set is a good choice to use at present. Using this data and suitable software it is possible to calculate the moment of perihelion for any give orbit.
Here are a few perihelion dates for the Earth
4th January 2014 at 11:59UTC
4th January 2015 at 06:37UTC
2nd January 2016 at 22:49UTC
4th January 2017 at 14:18UTC