Timezone issues returning a date object to a Google Sheets spreadsheet - date

Question

I'm writing a custom function to return financial quotes obtained from Yahoo Finance to a Google Sheets spreadsheet (the built-in GoogleFinance() won't cut it for various reasons), and I'm having problems to return a Date object containing the date and time of the quote. https://developers.google.com/apps-script/guides/sheets/functions says that "Times and dates in Sheets become Date objects in Apps Script. If the spreadsheet and the script use different time zones (a rare problem), the custom function will need to compensate", but it's not clear how.
First of all, the timezone of my browser is "asia/hong_kong", i.e. GMT+0800 (HKT). It turns out that the script somehow knows that, because when I format a Date object built with d = new Date(2016,5,24,0,0,0,0) with d.toString() I get "Wed Jun 24 2016 00:00:00 GMT+0800 (HKT)". This is weird, because SpreadsheetApp.getActiveSpreadsheet().getSpreadsheetTimeZone() actually returns "America/Los_Angeles", indicating that the server is on the West Coast of the US: so why does the method toString(), which is run on the server side, format the date according to the Hong Kong time? This sounds like a bug to me.
Anyway, the biggest problem I'm having is that when the custom function returns the date object to the spreadsheet, if I format the cell with Format -> Number -> Date time, what I see displayed is "23/06/2016 09:00:00", i.e. the date and time on the US West Coast when the Hong Kong date and time are the ones I specified through the parameters passed to the constructor (corresponding to Wed Jun 24 2016 00:00:00).
Then I tried to build the Date object with new Date(Date.UTC(2016,5,24,0,0,0,0)), but (predictably enough) now the spreadsheet displays "23/06/2016 17:00:00", which is the date and time on the West Coast when the UTC date and time are the ones I specified.
So, how do I get a Date object that, once returned to the spreadsheet, shows exactly the same date and time I used to build it? I tried to change the spreadsheet's timezone to UTC using ss.setSpreadsheetTimeZone("Etc/UTC"), but then unfortunately the script returns the error "You do not have permission to call setSpreadsheetTimeZone"...
This is the kludgy solution I managed to write (but I hope there is a better way):
// hours and minutes contain values obtained from the text quote; the date is in the string variable:
// date_str = "05/24/2016";
var tz = SpreadsheetApp.getActiveSpreadsheet().getSpreadsheetTimeZone(); // this returns the TZ of the server, "America/Los_Angeles"
var ums = Date.parse(date_str+" UTC");
var offset = Date.parse(Utilities.formatDate(new Date(ums), tz, "MM/dd/yyyy HH:mm:ss") + " UTC") - ums; // time in LA - UTC time in ms
var d = new Date(Date.parse(date_str));
d = new Date(Date.UTC(d.getFullYear(),d.getMonth(),d.getDate(),hours,minutes,d.getSeconds())-offset);
return d;