Sylvanus G. Morley reports that the Maya could calculate the solar year with accuracy similar to modern astronomy.
Sylvanus G. Morley, The Ancient Maya, 3rd ed. (Stanford, CA.: Stanford University Press, 1956), 429
Using the simplest of equipment, the Maya calculated the length of the solar year with an accuracy equal to that of modern astronomy, and devised correction formulae to adjust the discrepancy between the true year and the calendar year which is handled by our leap-year correction. They worked out an accurate lunar calendar and calculated the synodical revolutions of Venus, in each case devising means for correcting the accumulated error.