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Calculator does the following:
output.SetValue(Date.UTC( input.getFullYear(), input.getMonth(), input.getDate(), input.getHours(), input.getMinutes(), input.getSeconds() ));
As you can see, instead of simple getTime call on Date object, which gives us milliseconds in UTC for local time (using browser's timezone), it creates UTC Date, which gives us milliseconds in UTC for UTC time.