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Why did the Mayan calendar use 18 times 20 rather than 20 squared in one place value?

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Two vertical tablets display ancient symbols for the numbers one and ten from Egyptian hieroglyphic, Egyptian hieratic, Sumerian, and Cretan scripts, with each symbol shown in red next to its culture and approximate date. — numerals and numeral systems Examples of ancient symbols for the numbers 1 and 10.

The Maya used a modified vigesimal (base-20) place-value system for their Long Count calendar to align their mathematical units with their 365-day year.1

While the system is based on a count of 20, the Maya substituted 18 for 20 as the multiplier for the second order of units. This adjustment was made to create the tun, a period consisting of 18 uinals (each of 20 days), which equals 360 days. By establishing this value, the Maya created a unit that closely approximates the solar year of 365 days, making it more practical for tracking time and historical records than a strict base-20 system, which would have resulted in a third-place value of 400 days.12

Although this modification allowed for a better approximation of the solar year, it introduced complexity into their calculations, as it made multiplication and division more difficult compared to a standard positional system.1