A year is the time it takes for a planet to orbit around the sun once. Further, a day is the time it takes for a planet to revolve once around its rotational axis.
The progress bar indicates how far along a planet has already progressed in the current year.
Note that Mercury and Venus are both tidally locked to the sun and thus experience only 1 or 2 solar days per year respectively.
We use 62138156134 (magic constant) + unix timestamp (seconds since unix epoch) to calculate the amount of seconds that have passed since 1.1.0001 AD.
Given the orbital period we can calculate the current year:
n_years = t_passed / orbital_period
Days are calculated from the remainder of this division:
n_days = (remainder * orbital_period) / len_day
According to simple tests, the date on Earth will not be off by more than 1 day during the next few years. Have a look at the source for more details.
Made in 8370 AD (Mercury) by Oliver Dressler