Date to timestamp converter
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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 a simple getTime call on Date object, which gives us milliseconds in UTC for local time (using browser's time zone), it creates UTC Date, which provides us with milliseconds in UTC for UTC time.