Complex sentence: Therefore, only certain border cities in Mexican states that observe DST follow the same pattern as Canada and the United States while the remainder of Mexico is out of phase, beginning DST on the first Sunday of April and ending on the last Sunday of October.
Split into simpler sentences: Therefore, only certain border cities in Mexican states that observe DST follow the same pattern as Canada and the United States. The rest of Mexico remains out of phase, beginning DST on the first Sunday of April and ending on the last Sunday of October.