Google Apps Script - Adding n Hours to Current Time and Date

Question

I am new to the Google Apps Script code and am trying to simply add a user defined number of hours to the current time (lets say 12 hours for example). The code would then insert the time and date 12 hours into the future within a google doc.

I use a ui.prompt to have the user enter the number of hours into the future they want. The code I have does not give me an error, but it puts in the current hour some strange number of days into the future (not sure why it is doing this). This is the code I have and the else section is where I am running into issues...

function UpdateDocument() {

  var document = DocumentApp.getActiveDocument();
  var date = new Date();
  var tomorrow = new Date();
  tomorrow.setDate(tomorrow.getDate()+1);
  var ui = DocumentApp.getUi();

var regExpUpdateDate = "[A-Z]{3}, [A-Z]{3} [A-Z]{1}, [A-Z]{4}"; // Regular expression to find the correct line for the Next Update

  // Ask User if the they want to include the Next Update Line  
  var response = ui.alert('Would you like to include the Next Update line?' , ui.ButtonSet.YES_NO);

  // Process the users response.
  if (response == ui.Button.YES) {
    var responseNextUpdate = ui.prompt('Enter the number of hours you want until the next update (i.e. 12, 24, etc.)' 
                                       + ' or leave blank if you only want to include the date and omit the time. Note that'
                                       + ' leaving it blank will default to the day 24 hours from now.'); // Prompts user for number of hours until the next update
    if (responseNextUpdate.getResponseText() == '') {
      document.replaceText(regExpUpdateDate, Utilities.formatDate(tomorrow, 'America/Denver', 'EEEE, MMMM d, yyyy'));
    }
    else { // This is the section that I am having issues with...
      var userSelectedHours = new Date();
      userSelectedHours.setDate(userSelectedHours.getHours() + 2);
      document.replaceText(regExpUpdateDate, Utilities.formatDate(userSelectedHours, 'America/Denver', 'h a EEEE, MMMM d, yyyy'));
    }
  }
}

Answer 1

I find it easier to use milliseconds to add/subtract time.

  var now = new Date(); // Fri Jul 05 09:53:12 GMT+05:30 2019

  var nowInMS = now.getTime(); // 1562300592245

  var add = 12 * 60 * 60 * 1000; // 43200000 = 12 hours in milliseconds

  var twelveHoursLater = nowInMS + add; // 1562343792245

  var futureDate = new Date(twelveHoursLater); // Fri Jul 05 21:53:12 GMT+05:30 2019

  var futureDateFormatted = Utilities.formatDate(futureDate, Session.getScriptTimeZone(), "dd-MMM-yy hh:mm a"); // 05-Jul-19 09:53 PM