periodStart property
The billed period. Both carry omitempty and neither is ever empty, which looks contradictory and is not: the shape they reproduce is a time value, and omitempty does nothing to a struct — so those keys render even for the zero instant. The adapter formats the zero instant rather than skipping it, which is what keeps the two wires the same.
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? periodStart;