Custom date validation in google sheets with popup

I have a google sheet which is edited everyday by over 20 people. And sometimes it happens that someone paste something other than a date in "Date Column" or just simple write date in incorrect format. So I am looking to make a script which set date format to "yyy-mm-dd", then set data validation to check if date is correct (to avoid situation when you have a date like "2017-22-17"), and at the end popup message box if you have pasted or wrote incorrect date. I wrote below code and triggered it "onEdit" but I cannot handle this popup message thing ;/

  function SetDateFormat() {
  var ss = SpreadsheetApp.getActiveSpreadsheet()
  var sheet = ss.getSheetByName("Sheet1")
  var cell = sheet.getRange(4, 3, sheet.getMaxRows())
  cell.setNumberFormat('yyyy-mm-dd')
}


function setAndCheckDate() {
  var ss = SpreadsheetApp.getActive()
  var sheet =ss.getSheetByName("Sheet1")
  var cell = sheet.getRange(4, 3, sheet.getMaxRows())
  var rule1 = cell.getDataValidation()
      var rule2 =SpreadsheetApp.newDataValidation().requireDate().build()
      cell.setDataValidation(rule2)

      if(rule1 !== rule2) {
        Browser.msgBox("Date is incorrect!")
      }
}

Another thing is that validating script doesn't work while I'm pasting cell with incorrect data to my date column.

https://docs.google.com/spreadsheets/d/1ZPbfX60E46W95XhMTXpBSLTUAT71QCe-MFnYhYy2PPw/edit?usp=sharing

Can you advise?

You're looking for the formatDate(date, timeZone, format) in Apps Script which formats date according to specification you described.

 // This formats the date as Greenwich Mean Time in the format
 // year-month-dateThour-minute-second.
 var formattedDate = Utilities.formatDate(new Date(), "GMT", "yyyy-MM-dd'T'HH:mm:ss'Z'");
 Logger.log(formattedDate);