An mlife "year" runs from October 1 to September 30. Whatever tier you reach during that year, you remain that tier for the rest of that year and the next
I received a new card and it says Sept 2018 on the bottom, so I should stay gold until at least then?
Correct. You have until 9/30 to try to make Platinum. At 10/1 your points will reset to 0, but your status will remain gold until 9/30/18.