Dates (Duration) in Google Sheets Script - date

Question

I have a Google Sheet with three columns:
- Date and time (timestamp)
- Duration
- Description
I have an script that when I write something in 'Description', inserts in 'Date' the date and time at this moment, and the 'Duration':
function onEdit(e) {
if(e.source.getActiveSheet().getName() == "Sheet2" ) {
var col = e.source.getActiveCell().getColumn();
if(col == 3 ) {
// I'm in column three
var cellTimeStamp = e.range.offset(0,-2); // First column of the same row
var cellTimeDiff = e.range.offset(0,-1); // Second column of the same row
var cellTimePrev = e.range.offset(-1,-2); // First column of the previous row
var timeTimeStamp = new Date();
var iniTime = cellTimePrev.getValue().getTime();
var finTime = timeTimeStamp.getTime() ;
var timeDiff = String(finTime - iniTime) ;
cellTimeStamp.setValue(timeTimeStamp);
cellTimeDiff.setValue(timeDiff); // [***]
}
}
}
When this executes (as an event) in the column of 'Duration' there is NOT something in the format of 'HH:mm:ss'.
But if I remove the last line in this script and adds this formulae in the sheet:
=A3-A2 (in row 3)
=A4-A3 (in row 4)
...
then it works ok.
I'd like to know how to meet the same result but with a script.
Thanks in advance.

Answer 1

timeDiff is the result of finTime - iniTime which are both native date object values, which means we have milliseconds .
converting that in hh:mm:ss is simple math... : 60 seconds in a minute and 60 minutes in an hour...
A simple code could be like this :
function msToTime(s) {
var ms = s % 1000;
s = (s - ms) / 1000;
var secs = s % 60;
s = (s - secs) / 60;
var mins = s % 60;
var hrs = (s - mins) / 60;
return hrs + ':' + mins + ':' + secs; // milliSecs are not shown but you can use ms if needed
}
If you prefer formating your string more conventionally (2 digits for each value) don't forget you can use Utilities.formatString() to do so.
example below :
return Utilities.formatString("%02d",hrs) + ':' + Utilities.formatString("%02d",mins) + ':' + Utilities.formatString("%02d",secs);
EDIT
Following your comment :
Spreadsheets are smarter than you think, you can try the code below and you will see that the result is actually a time value.(check by double clicking on it)
function test() {
var sh = SpreadsheetApp.getActiveSheet();
var t1 = sh.getRange('a1').getValue().getTime();
var t2 = sh.getRange('b1').getValue().getTime();
sh.getRange('c1').setValue(msToTime(t1-t2)).setNumberFormat('hh:mm:ss');
}
function msToTime(s) {
var ms = s % 1000;
s = (s - ms) / 1000;
var secs = s % 60;
s = (s - secs) / 60;
var mins = s % 60;
var hrs = (s - mins) / 60;
return hrs + ':' + mins + ':' + secs; // milliSecs are not shown but you can use ms if needed
}
note that setNumberFormat('hh:mm:ss') is optional, it's only there to force the spreadsheet to display hour:min:sec format but automatic mode works as well.