I arrived in Guatemala City a week ago. Two days ago I was stopped at a routine traffic checkpoint where I presented my US driver's license. The officer informed me that I am required to carry my passport with me while driving, to show my date of entry into Guatemala, because I am only permitted to drive with a foreign license for 30 days. I was not fined, rather only told to carry my passport in the vehicle in the future.
My girlfriend thinks he just made that up.
What are the relevant rules?