hide_rows.gs

const MILLIS_PER_DAY = 1000*60*60*24
const HEADER_ROWS = 1

function hide() {
  const ss    = SpreadsheetApp.getActiveSpreadsheet()
  const sheet = ss.getSheets()[0]
  const dateCol  = sheet.getRange("A:A").getValues().map(function(row) {
    return row[0]
  })

  var i           = HEADER_ROWS
  while (_daysBetween(dateCol[i], Date.now()) > 20) {
    i++ 
  }
  sheet.hideRows(HEADER_ROWS + 1, i)
}

function _daysBetween(date1, date2){ 
  return Math.floor((date2 - date1) / MILLIS_PER_DAY)
}