 During this period, Lounsbury studied the Venus almanac in the Dresden codex and concluded that the Thompson correlation fits the evidence in the codex better than the standard GMT correlation.A correlation constant is the number of days between the start of the Julian Period (January 1, 4713 BCE) and the era date of the Long Count of 13.0.0.0.0 4 Ajaw 8 Kumk'u.It is used to convert between the Long Count and western calendars.