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startDayOfYear (http://rs.tdwg.org/dwc/terms/startDayOfYear)

@return [String]
Seasonal temporal subcontext within the eventDate context. Useful for
migratory species. The earliest ordinal day of the year on which the
distribution record is valid. Numbering starts with 1 for January 1 and
ends with 365 (or 366 if it is a leap year) for December 31.

Example: "90"
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