Google Spreadsheet macro example

macro.gs

/** @OnlyCurrentDoc */

const startDate = new Date("2021-06-16")
// so the first one is D2:K2
function MarkEmptyCellsInYesterdayRowWithHyphen() {
  const sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Group 2 - B");
  // Activates the sheet 
  SpreadsheetApp.setActiveSheet(sheet);
  
  const today = new Date()
  const diffInDays = days_between(today, startDate)
  // Today, Jul 2 is 17, so tomorrow it will be 18.
  // We only want to do it for the day before
  // But since the first row in the sheet is the column 
  // it means July 2 is actually row 18
  // Example: Today is July 2nd, 2021.
  // There are 17 days between today and the start day
  // If we were to run as is and use that number (17) for the row
  // it would update 07/01/2021 because that's row 17
  // Long story short, this works
  const rowForDayBefore = `D${diffInDays}:K${diffInDays}`
  const range = sheet.getRange(rowForDayBefore);
  sheet.setActiveRange(range);
  // Grab current row
  const values = range.getValues()
  // Update the empty cells with "-"
  const updatedValues = updateValues(values)
  range.setValues(updatedValues)
};

function updateValues(values) {
  // If the value is empty, we mark it with "-"
  // It looks like [ [ 'x', 'x', 'x', '', '', 'x', 'x', 'x' ] ]
  // which is why we return in array, 2d array
  return [values[0].map(v => v === "" ? "-" : v)]
}

// source: https://stackoverflow.com/a/2627482/3015595
function days_between(date1, date2) {
    // The number of milliseconds in one day
    const ONE_DAY = 1000 * 60 * 60 * 24;
    // Calculate the difference in milliseconds
    const differenceMs = Math.abs(date1 - date2);
    // Convert back to days and return
    return Math.round(differenceMs / ONE_DAY);
}