Find how much free time (i.e., unscheduled) there is per calendar day

FreeTime.gs

/*
 * User Settings
 */
// Always use current week. How many weeks past that to check?
const WEEKS_AHEAD = 0;

// Assume each day starts, ends at the same times
const WORK_START_HR = 9;
const WORK_END_HR = 18;


// Start date is adjusted to be the beginning of the same week
const WEEK_START = new Date();
WEEK_START.setDate(WEEK_START.getDate() - WEEK_START.getDay() + 1);  // add 1 to move from Sunday to Monday
WEEK_START.setHours(WORK_START_HR, 0, 0, 0);

// End date is adjusted to be the end of the week beginning with WEEK_START
const WEEK_END = new Date(WEEK_START);
WEEK_END.setDate(WEEK_START.getDate() + 4);  // Monday + 4 -> Friday
WEEK_END.setHours(WORK_END_HR, 0, 0, 0);


/*
 * Global Constants
 */
const DAY_MS = 24 * 60 * 60 * 1000;
const MIN_SLOT_MIN = 15;
const SLOTS_PER_HR = 60 / MIN_SLOT_MIN;

const HOURS_PER_DAY = WORK_END_HR - WORK_START_HR;
const SLOTS_PER_DAY = HOURS_PER_DAY * SLOTS_PER_HR;


/*
 * Functions
 */

function _workdays(earlier, later) {
  const start = new Date(earlier);
  start.setHours(0, 0, 0, 0);
  
  const end = new Date(later);
  end.setHours(0, 0, 0, 0);

  const diff = end - start;
  const diffDays = diff / DAY_MS;
  var workdays = 0;
  if (diffDays < 0) {
    workdays = Math.ceil(diffDays) - 1;
  } else {
    workdays = Math.floor(diffDays) + 1;
  }
  return workdays;
}


function _slotIdx(eventTime, start) {
  const workdays = _workdays(start, eventTime);
  if (workdays < 0) {
    return workdays;
  }
  const daysFromStart = workdays - 1;

  const hoursFromStart = Math.min(
    HOURS_PER_DAY,
    Math.max(0, eventTime.getHours() - WORK_START_HR)
  );
  const offset = Math.floor(eventTime.getMinutes() / MIN_SLOT_MIN);
  const startIdx =
    (daysFromStart * SLOTS_PER_DAY)
    + (hoursFromStart * SLOTS_PER_HR)
    + offset;
  return startIdx;
}


function _initFree(start, end) {
  const workdays = _workdays(start, end);
  const totalWorkHours = workdays * HOURS_PER_DAY;
  const freeSlots = new Array(totalWorkHours * SLOTS_PER_HR).fill(1);
  return freeSlots;
}


function _blockBusy(workSlots, weekStart, weekEnd) {
  const calId = Session.getActiveUser().getEmail();
  const freeBusy = Calendar.Freebusy.query({
    "timeMin": weekStart.toISOString(),
    "timeMax": weekEnd.toISOString(),
    "items": [{
      "id": calId,
      "busy": "Active"
    }]
  });

  Logger.log('Using calendar "' + calId + "' between " + weekStart + " and " + weekEnd);

  for (const busy of freeBusy.calendars[calId].busy) {
    const startIdx = Math.max(0, _slotIdx(new Date(busy.start), weekStart));
    const endIdx = _slotIdx(new Date(busy.end), weekStart);
    workSlots.fill(0, startIdx, endIdx);
  }
}


function _printByDay(timeSlots, weekStart, weekEnd) {
  const day = new Date(weekStart);
  const workdays = _workdays(weekStart, weekEnd);
  const freeStats = [];

  for (var i = 0; i < workdays; i++) {
    const start = i * SLOTS_PER_DAY;
    const workdaySlots = timeSlots.slice(start, start + SLOTS_PER_DAY);
    const freeSlots = workdaySlots.reduce((a, b) => a + b);

    freeStats.push({
      date: new Date(day).toDateString(),
      total: HOURS_PER_DAY,
      free: freeSlots / SLOTS_PER_HR
    });
    
    day.setDate(day.getDate() + 1);
  }
  return freeStats;
}


/*
 * Main Method
 */
function calculateUnscheduledHours() {
  const _start = new Date(WEEK_START);
  const _end = new Date(WEEK_END);

  for (var weekIdx = 0; weekIdx <= Math.max(0, WEEKS_AHEAD); weekIdx++) {
    const freeSlots = _initFree(_start, _end);
    _blockBusy(freeSlots, _start, _end);
    const stats = _printByDay(freeSlots, _start, _end);
    var free = 0;
    var total = 0;
    for (const stat of stats) {
      free += stat.free;
      total += stat.total;
      Logger.log(stat.date + ": " + stat.free + "hrs @ " + ((stat.free / stat.total) * 100).toLocaleString() + "%");
    }
    Logger.log(free + " of " + total + "hrs free (" + ((free / total) * 100).toLocaleString() + "%)");

    _start.setDate(_start.getDate() + 7);
    _end.setDate(_end.getDate() + 7);
  }
}