How to use the @fullcalendar/core.startOfDay function in @fullcalendar/core

export function getTimeGridLine(date) { // not in Scheduler
  date = ensureDate(date)

  var day = startOfDay(date)
  var timeMs = date.valueOf() - day.valueOf()
  var top = getTimeGridTop(timeMs)
  var dayEls = getTimeGridDayEls(date)
  var dayRect

  expect(dayEls.length).toBe(1)
  dayRect = getBoundingRect(dayEls.eq(0))

  return {
    left: dayRect.left,
    right: dayRect.right,
    top: top,
    bottom: top
  }
}

renderSlatRowHtml(dateProfile: DateProfile) {
    let { dateEnv, theme, isRtl } = this.context
    let html = ''
    let dayStart = startOfDay(dateProfile.renderRange.start)
    let slotTime = dateProfile.minTime
    let slotIterator = createDuration(0)
    let slotDate // will be on the view's first day, but we only care about its time
    let isLabeled
    let axisHtml

    // Calculate the time for each slot
    while (asRoughMs(slotTime) < asRoughMs(dateProfile.maxTime)) {
      slotDate = dateEnv.add(dayStart, slotTime)
      isLabeled = wholeDivideDurations(slotIterator, this.labelInterval) !== null

      axisHtml =
        '' +
          (isLabeled ?
            '<span>' + // for matchCellWidths
              htmlEscape(dateEnv.format(slotDate, this.labelFormat)) +</span>

function computeEventMutation(hit0: Hit, hit1: Hit, massagers: eventDragMutationMassager[]): EventMutation {
  let dateSpan0 = hit0.dateSpan
  let dateSpan1 = hit1.dateSpan
  let date0 = dateSpan0.range.start
  let date1 = dateSpan1.range.start
  let standardProps = {} as any

  if (dateSpan0.allDay !== dateSpan1.allDay) {
    standardProps.allDay = dateSpan1.allDay
    standardProps.hasEnd = hit1.component.context.options.allDayMaintainDuration

    if (dateSpan1.allDay) {
      // means date1 is already start-of-day,
      // but date0 needs to be converted
      date0 = startOfDay(date0)
    }
  }

  let delta = diffDates(
    date0, date1,
    hit0.component.context.dateEnv,
    hit0.component === hit1.component ?
      hit0.component.largeUnit :
      null
  )

  if (delta.milliseconds) { // has hours/minutes/seconds
    standardProps.allDay = false
  }

  let mutation: EventMutation = {

function computeEventMutation(hit0, hit1, massagers) {
    var dateSpan0 = hit0.dateSpan;
    var dateSpan1 = hit1.dateSpan;
    var date0 = dateSpan0.range.start;
    var date1 = dateSpan1.range.start;
    var standardProps = {};
    if (dateSpan0.allDay !== dateSpan1.allDay) {
        standardProps.allDay = dateSpan1.allDay;
        standardProps.hasEnd = hit1.component.opt('allDayMaintainDuration');
        if (dateSpan1.allDay) {
            // means date1 is already start-of-day,
            // but date0 needs to be converted
            date0 = startOfDay(date0);
        }
    }
    var delta = diffDates(date0, date1, hit0.component.dateEnv, hit0.component === hit1.component ?
        hit0.component.largeUnit :
        null);
    if (delta.milliseconds) { // has hours/minutes/seconds
        standardProps.allDay = false;
    }
    var mutation = {
        datesDelta: delta,
        standardProps: standardProps
    };
    for (var _i = 0, massagers_1 = massagers; _i < massagers_1.length; _i++) {
        var massager = massagers_1[_i];
        massager(mutation, hit0, hit1);
    }

export function getTimeGridPoint(date) {
  date = ensureDate(date)

  var day = startOfDay(date)
  var timeMs = date.valueOf() - day.valueOf()
  var top = getTimeGridTop(timeMs)
  var dayEls = getTimeGridDayEls(date)
  var dayRect

  expect(dayEls.length).toBe(1)
  dayRect = getBoundingRect(dayEls.eq(0))

  return {
    left: (dayRect.left + dayRect.right) / 2,
    top: top
  }
}

computeDateTop(when: DateMarker, startOfDayDate?: DateMarker) {
    if (!startOfDayDate) {
      startOfDayDate = startOfDay(when)
    }
    return this.computeTimeTop(createDuration(when.valueOf() - startOfDayDate.valueOf()))
  }

function computeDateVars(dateProfile) {
    var dayStart = startOfDay(dateProfile.renderRange.start);
    var viewEnd = dateProfile.renderRange.end;
    var dayDates = [];
    var dayRanges = [];
    while (dayStart < viewEnd) {
        dayDates.push(dayStart);
        dayRanges.push({
            start: dayStart,
            end: addDays(dayStart, 1)
        });
        dayStart = addDays(dayStart, 1);
    }
    return { dayDates: dayDates, dayRanges: dayRanges };
}

function computeDateVars(dateProfile: DateProfile) {
  let dayStart = startOfDay(dateProfile.renderRange.start)
  let viewEnd = dateProfile.renderRange.end
  let dayDates: DateMarker[] = []
  let dayRanges: DateRange[] = []

  while (dayStart < viewEnd) {

    dayDates.push(dayStart)

    dayRanges.push({
      start: dayStart,
      end: addDays(dayStart, 1)
    })

    dayStart = addDays(dayStart, 1)
  }