How to use the @govuk-pay/pay-js-commons.datetime function in @govuk-pay/pay-js-commons

const model = {
    name: req.user.username,
    serviceId: req.service.externalId,
    period,
    links,
    linksToDisplay,
    goLiveNotStarted: req.service.currentGoLiveStage === NOT_STARTED,
    goLiveStarted: goLiveStartedStages.includes(req.service.currentGoLiveStage),
    goLiveRequested: goLiveRequestedStages.includes(req.service.currentGoLiveStage),
    account: req.account
  }

  try {
    const { fromDateTime, toDateTime } = getTransactionDateRange(period)

    const transactionsPeriodString = `fromDate=${encodeURIComponent(datetime(fromDateTime, 'date'))}&fromTime=${encodeURIComponent(datetime(fromDateTime, 'time'))}&toDate=${encodeURIComponent(datetime(toDateTime, 'date'))}&toTime=${encodeURIComponent(datetime(toDateTime, 'time'))}`

    logger.info(`[${correlationId}] successfully logged in`)

    if (isADirectDebitAccount(gatewayAccountId)) {
      // todo implement transaction list for direct debit
      return response(req, res, 'dashboard/index', Object.assign(model, {
        activityError: true
      }))
    }

    const namespace = getNamespace(clsXrayConfig.nameSpaceName)
    const clsSegment = namespace.get(clsXrayConfig.segmentKeyName)

    AWSXRay.captureAsyncFunc('ledgerClient_getTransactionSummary', function (subsegment) {
      LedgerClient.transactionSummary(gatewayAccountId, fromDateTime, toDateTime, { correlationId: correlationId })
        .then(result => {