The time zone in mexico including Merida and Mexico City adheres  to the Central time in the US, which is same as in Dallas Texas. It is six hours behind the Greenwich Mean Time or GMT.

The second time zone starts at North of Puerto Vallarta but Puerto Vallarta itself is not affected. It only affects the areas on the coast north including the popular beach destinations of Punta de Mita and Mazatlan. All of Baja California Sur, including areas in Los Caboz, Loreto, La Paz, and Todos Santos.

Chihuahua City is not also affected by the time zone because it is too far east. This time zone actually adheres to Mountain Time like in Denver, CO and is one hour behind Mexico.