google-sheet-reminder.js

function remindify() {
  var ss = SpreadsheetApp.openById('my-google-sheet-id');
  var sheet = ss.getSheets()[0];
  var cols = sheet.getMaxColumns();
  var rows = sheet.getMaxRows();
  var range = sheet.getRange(1,1, rows, cols);
  var values = range.getValues();
  var to = '[email protected]';
  var now = new Date();
  var A_DAY_MS = 43200000;
  var diff;
  var val;
  var cell;
  
  for (var i = 0; i < values.length; i++) {
    if (i > 0) {
      if (values[i][0] && values[i][4] !== true) {
        diff = Math.abs(now - values[i][0]);
        if (diff <= A_DAY_MS) {
          MailApp.sendEmail(to, values[i][2], values[i][3]);
          cell = sheet.getRange(i + 1, 5);
          cell.setValue(true);
        }
      }
    }
  }
}