No Leap Day, and That Was Not an Oversight

The count contains no correction for the quarter day by which the solar year exceeds three hundred and sixty five, so it slips against the seasons at roughly one day every four years, and a given month arrives about a month earlier in the seasons every century and a bit. Mesoamerican astronomers were entirely capable of measuring the discrepancy, so the absence of a correction is a design decision rather than an error. The reason is that this count was not carrying the burden of tracking the sun.

Long Count notation supplied an absolute day tally that anchored everything, seasonal and agricultural timing could be observed directly, and the meshing of this count with the two hundred and sixty day cycle produced a fifty two year period whose integrity depends on both counts running without adjustment. Inserting a leap day would break the meshing, which is the thing the system exists to do. Whether any community ever applied a correction has been debated, with arguments made from month names that appear to have drifted out of their seasons and from colonial period records. No agreed evidence of a systematic adjustment has been produced, and the atlas records the count as uncorrected while noting that the question has been raised seriously.