September 2018 when you no longer lived in the home is the start of the 5 year period. You would need to sell the home on or before September 1, 2023.
You must have lived in and owned the home for at least two years in the five year period on the date of sale, you do not have to report the sale if your gains are less then the exclusion amounts of $250,000 if filing Single or $500,000 if filing Married Filing Jointly (and both lived in the home for two years).
