javascript - convert localtime to different timezone

Does anyone know how to 开发者_运维知识库convert "Date().getTime()" to UTC?

Obviously the getTime() is the users local time so I need a function to get the users timezone, convert the timestamp to UTC so I can then * it by -8 to get the current PST time.

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Date.getTimezoneOffset()

as seen here. It returns the user's browser time relative to GMT, which you'll compare to your server time, relative to GMT.