How to use the astronomia/lib/solstice.september2 function in astronomia

inYear (year) {
    let jde
    switch (this._season) {
      case 'march': {
        jde = solstice.march2(year, earth)
        break
      }
      case 'june': {
        jde = solstice.june2(year, earth)
        break
      }
      case 'september': {
        jde = solstice.september2(year, earth)
        break
      }
      case 'december': {
        jde = solstice.december2(year, earth)
        break
      }
    }

    const str = new julian.Calendar().fromJDE(jde).toDate().toISOString()
    let date
    if (/^[+-]\d{2}:\d{2}?$/.test(this._timezone)) { // for '+08:00' formats
      date = moment(str).utcOffset(this._timezone)
    } else { // for 'Asia/Shanghai' formats
      date = moment(str).tz(this._timezone) // move to timezone
    }