No the library doesn’t have any other output for ISO 3339
// Represent 18 September 2019 in ISO 3339 format, 2 digits of second fraction:
const result = formatRFC3339(new Date(2019, 8, 18, 19, 0, 52, 234), { fractionDigits: 2 })
//=> '2019-09-18T19:00:52.23Z'
ISO 8601 is closest to what is expected by the backend.
// Represent 18 September 2019 in ISO 8601 format (UTC):
const result = formatISO(new Date(2019, 8, 18, 19, 0, 52))
//=> '2019-09-18T19:00:52Z'