The chief evidence in favor of giving the 0 or 20 glyph the meaning of 20 is, that this glyph is often drawn with a hand stretching across its lower part, especially when the main part of the glyph is a face. Now the face glyphs which represent the cycle of 144,000 days and the katun of 7,200 days are very similar, except that the cycle glyph has also a hand across its lower part, and the cycle is equal to 20 katuns; but this evidence is somewhat weak, since it is clear that even if the 0 or 20 glyph should be decided to mean 20, in all calculations it is to be treated as 0, as is proved by many of the inscriptions of Palenque, Piedras Negras, Copan, and elsewhere.
On the whole, therefore, I think the weight of evidence is in favor of the hypothesis that the Mayas called the beginning days of their month Day 0 and numbered the days of their month from 0 to 19.