Historical clue formulations

• Like Nahuatl speakers — 2015-11-22
• Like many Mexicans' forebears — 2015-01-05
• Kind of calendar with 18 months — 2002-09-19
• Like some Mexican pyramids — 1999-04-29
• Like some pre-Columbian culture — 1998-09-08
• Uto-___ (language group) — 1998-02-27