Get browser timezone or time offset to UTC


we are porting a application to vaadin and need to know the browser timezone, or at least the time offset to UTC.
The reason is, that the users are allowed to enter date/times in the user local time zone, they should then on the server be converted into UTC timestamp and saved to the database.

When we then later on present the dates/times to another user,
we need to convert the utc timestamps into the users browser timezone.

Since we also sometimes show the dates/times as string (for example in PDF reports)
we need to do the timezone transformations ourself, we can’t use a date/time component for this.