The link to the documentation you referred to has been archived. According to a newer documentation link on App Store receipts, it documents these fields:original_purchase_datestringThe time of the original app purchase, in a date-time format similar to ISO 8601.original_purchase_date_msstringThe time of the original app purchase, in UNIX epoch time format, in milliseconds. Use this time format for processing dates.original_purchase_date_pststringThe time of the original app purchase, in the Pacific Time zone.