To change and start the counting 4 glyphs earlier than from Ga1-26 is not acceptable unless there is support from the messages delivered by the glyphs:

Ga1-21 is a glyph type which probably indicates end. Ga1-22 therefore is located at the beginning. In the calendar for the night this type of glyph arrives beyond midnight, announcing the birth of a new day.

Furthermore, Ga1-21--22 together suggest Ga7-19: