New to Formulas

Thanks in advance for your help!
I need to create a formula to determine a project’s classification based on Date Added. There are three classifications: Rocks – 90 days or less, Boulders – 180 days or less, and Mountains – 180 days or more.

This should be at least close to what you want:

IF(
   DATETIME_DIFF({Date Added}, TODAY(), 'days') >= 180,
   "Mountains",
   IF(
      DATETIME_DIFF({Date Added}, TODAY(), 'days') > 90,
      "Boulders",
      "Rocks"
   )
)