Thanks for driving this, @veronica!
Onset is left blank on promotion, with an optional month field […] Any objection to the optional month field suggestion?
No objection to blank-by-default, that sounds correct to me. On the month field specifically: our OpenmrsDatePicker is day-precision only (no month/year mode), so a month-only input is a custom control rather than a prop. And the backend stores onsetDate as a full timestamp, so a month gets persisted as the 1st of that month and the granularity isn’t retained. Month-only would be a UI convention we maintain, not something the data model honours. The Cheapest v1 of this feature will likely be the existing day picker (optional, blank); a real month picker can follow.