Quite honestly, it doesn't matter. As long as you put dates that will make sense in the context of your statements about your relationship, that's the important part.
Original Nationality: US
Visa #1: PMV (300) through Washington, D.C
Applied: April 2013.
Visa Granted: January 2014.
Visa #2: Subclass 820 (From PMV).
Applied: End of April 2014.
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PR Granted: Early April 2017.