Google Apps Script spreadsheet date manipulation

I have three scripts that are in a google docs spreadsheet. In this spreadsheet, in column H (or column 8), if I type an "x", the script changes it into that days date. After a few days, every date in column H has changed from a date to just a number. The numbers look like this: 40492, 40494, 40511. I am not sure what is causing this. Maybe it's something that is wrong in my script. I've pasted them below. Any ideas?

function onEdit(e) {

  var colorA = "yellow";
  var colorB = "#dddddd";
  var colorC = "#dddddd";
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Purchase Orders");
  var range = e.source.getActiveRange();
  var sheetName = SpreadsheetApp.getActiveSheet().getName();

  if (sheetName == "Purchase Orders") {

 // 3 is column C 
    if (range.getColumn() == 3 && range.getValue() != "") {
      sheet.insertRowAfter(range.getRow());
      var r = range.getRow() + 1;
      sheet.getRange("A" + r + ":H" + r).setBackgroundColor(colorC);
    }
  }

    var col = e.source.getActiveRange().getColumn();
    if(col == 3 || col == 8) {
    var rows = sheet.getMaxRows();

    //column C
    var rangeC = sheet.getRange("C1:C"+rows);
    var valuesC = rangeC.getValues();
    //column H range
    var rangeH = sheet.getRange("H1:H"+rows);
    var colorH = rangeH.getBackgroundColors();
    var valuesH = rangeH.getValues();

    //iterate over each row in column C and H
    //then change color
        for (var row = 0; row < valuesC.length; row++) {
          //check for columnC and column H
          var hRow = colorH[row];
          if (valuesC[row][0] != "" && valuesH[row][0] == "") {
            hRow[0] = colorA;
          } else if (valuesH[row][0] != "") {
            hRow[0] = colorB;
      }
        }
    sheet.getRange("H1:H" + rows).setBackgroundColors(colorH);




}
  }

And this one

function onEdit(e) {
  var ss = e.source.getActiveSheet();
  var r = e.source.getActiveRange();
  // 1 is A, 2 is B, ... 8 is H
  if (r.getColumn() == 8 && r.getValue() == "x") {
    r.setValue(Utilities.formatDate(new Date(), "MST", "yyyy-MM-开发者_如何学编程dd"));

  }  
}  

And this last one

ss = SpreadsheetApp.getActiveSpreadsheet();


function onOpen() {
 var ss = SpreadsheetApp.getActiveSpreadsheet();
  var menuEntries = [ {name: "New PO", functionName: "NewPO"}];
   ss.addMenu("New PO", menuEntries);
 }


function NewPO() {
  SpreadsheetApp.getActiveSheet().insertRowsBefore(1,6);


  // Adjust this range accordingly, these are the cells that will be
  // copied.  Format is getRange(startRow, startCol, numRows, numCols)
  ss.getSheetByName("PO Form").getRange(1, 1, 6, 8)
      .copyTo(SpreadsheetApp.getActiveSheet().getRange(1, 1, 6, 8));
   }

In OnEdit, you probably want to set the format for that cell as well. setNumberFormat(numberFormat) appears to be the function you are after.

http://code.google.com/googleapps/appsscript/class_range.html#setNumberFormat