How to read the correct time/duration values from Google Spreadsheet

Expanding on what Serge did, I wrote some functions that should be a bit easier to read and take into account timezone differences between the spreadsheet and the script.

function getValueAsSeconds(range) {
  var value = range.getValue();

  // Get the date value in the spreadsheet's timezone.
  var spreadsheetTimezone = range.getSheet().getParent().getSpreadsheetTimeZone();
  var dateString = Utilities.formatDate(value, spreadsheetTimezone, 
      'EEE, d MMM yyyy HH:mm:ss');
  var date = new Date(dateString);

  // Initialize the date of the epoch.
  var epoch = new Date('Dec 30, 1899 00:00:00');

  // Calculate the number of milliseconds between the epoch and the value.
  var diff = date.getTime() - epoch.getTime();

  // Convert the milliseconds to seconds and return.
  return Math.round(diff / 1000);
}

function getValueAsMinutes(range) {
  return getValueAsSeconds(range) / 60;
}

function getValueAsHours(range) {
  return getValueAsMinutes(range) / 60;
}

You can use these functions like so:

var range = SpreadsheetApp.getActiveSheet().getRange('A1');
Logger.log(getValueAsHours(range));

Needless to say, this is a lot of work to get the number of hours from a range. Please star Issue 402 which is a feature request to have the ability to get the literal string value from a cell.

There are two new functions getDisplayValue() and getDisplayValues() that returns the datetime or anything exactly the way it looks to you on a Spreadsheet. Check out the documentation here

The value you see (Sat Apr 12 07:09:21 GMT+00:09 1902) is the equivalent date in Javascript standard time that is 20000 hours later than ref date.

you should simply remove the spreadsheet reference value from your result to get what you want.

This code does the trick :

function getHours(){
  var sh = SpreadsheetApp.getActiveSpreadsheet();
  var cellValue = sh.getRange('E1').getValue();
  var eqDate = new Date(cellValue);// this is the date object corresponding to your cell value in JS standard 
  Logger.log('Cell Date in JS format '+eqDate)
  Logger.log('ref date in JS '+new Date(0,0,0,0,0,0));
  var testOnZero = eqDate.getTime();Logger.log('Use this with a cell value = 0 to check the value to use in the next line of code '+testOnZero);
  var hours = (eqDate.getTime()+ 2.2091616E12 )/3600000 ; // getTime retrieves the value in milliseconds, 2.2091616E12 is the difference between javascript ref and spreadsheet ref. 
  Logger.log('Value in hours with offset correction : '+hours); // show result in hours (obtained by dividing by 3600000)
}

note : this code gets only hours , if your going to have minutes and/or seconds then it should be developped to handle that too... let us know if you need it.

EDIT : a word of explanation...

Spreadsheets use a reference date of 12/30/1899 while Javascript is using 01/01/1970, that means there is a difference of 25568 days between both references. All this assuming we use the same time zone in both systems. When we convert a date value in a spreadsheet to a javascript date object the GAS engine automatically adds the difference to keep consistency between dates.

In this case we don't want to know the real date of something but rather an absolute hours value, ie a "duration", so we need to remove the 25568 day offset. This is done using the getTime() method that returns milliseconds counted from the JS reference date, the only thing we have to know is the value in milliseconds of the spreadsheet reference date and substract this value from the actual date object. Then a bit of maths to get hours instead of milliseconds and we're done.

I know this seems a bit complicated and I'm not sure my attempt to explain will really clarify the question but it's always worth trying isn't it ?

Anyway the result is what we needed as long as (as stated in the comments) one adjust the offset value according to the time zone settings of the spreadsheet. It would of course be possible to let the script handle that automatically but it would have make the script more complex, not sure it's really necessary.

For simple spreadsheets you may be able to change your spreadsheet timezone to GMT without daylight saving and use this short conversion function:

function durationToSeconds(value) {
  var timezoneName = SpreadsheetApp.getActive().getSpreadsheetTimeZone();
  if (timezoneName != "Etc/GMT") {
    throw new Error("Timezone must be GMT to handle time durations, found " + timezoneName);
  }
  return (Number(value) + 2209161600000) / 1000;
}

Eric Koleda's answer is in many ways more general. I wrote this while trying to understand how it handles the corner cases with the spreadsheet timezone, browser timezone and the timezone changes in 1900 in Alaska and Stockholm.