Because the kin is a single day and the Long Count is a plain tally of kins from a fixed origin, converting a Maya date into a European one reduces to a single unknown: how many days lie between the Long Count origin and some fixed point in the Julian day count. That number is the correlation constant, and choosing it correctly is the whole of the problem.
The value in general use is 584,283, the Goodman, Martinez and Thompson correlation, usually shortened to the GMT correlation, which places the Long Count origin 13.0.0.0.0, 4 Ajaw 8 Kumku, in August of 3114 BCE in the proleptic Gregorian calendar. It is supported by several independent lines of evidence, among them the alignment of Classic eclipse and Venus records with computed astronomy, the continuity of the 260-day count kept by highland Maya daykeepers, and radiocarbon dates from dated monuments and lintels.
A small number of competing constants have been proposed and continue to be argued for, differing by a few days or by rather more, and the debate is a live one rather than a settled point of history. What is not in dispute is the structure being correlated: the count of kins itself is unambiguous, and any correlation shifts every Maya date by the same amount.