> Luckily, the browser already knows what timezone the user is currently in
Incorrect (though usually true, I'll grant). I've often seen people with older Windows laptops (newer Windows variants, 7 and 8 certianly Vista maybe, are much better at this) that are set to display UK time but think that they are in UTC-0700. This can be fun when using file synchronisation tools... If the OS doesn't know its timezone then the browser won't either. This is a moot point when you are talking to just one user, until that user fixes their clock settings of course, but becomes a significant issue as soon as you need to coordinate timed actions between users.
Detecting the local time offset from your reference clock isn't always useful either: the machine's clock could be completely wrong so it says 01:23 where the user's wall-clock says 21:00 and your reference time matches neither.
Also some users may wish to fix your interface to a timezone other than the one currently set on the device they are currently using. Someone on holiday or on a business trip might want to keep some things linked to local time back home so they find it easier to coordinate with people there. Conversely someone currently in their home timezone might want to force your interface to match a non-local timezone for coordinating with teams/friends elsewhere in the world. You can't rely on them adjusting their clock in this circumstance as they may not be using their own machine and so might not be able to adjust the time/timezone settings (or may wish to keep them accurate to current local for other reasons).
tl;dr: Dealing with time values when there are any international concerns it harder than we tend to assume.
Incorrect (though usually true, I'll grant). I've often seen people with older Windows laptops (newer Windows variants, 7 and 8 certianly Vista maybe, are much better at this) that are set to display UK time but think that they are in UTC-0700. This can be fun when using file synchronisation tools... If the OS doesn't know its timezone then the browser won't either. This is a moot point when you are talking to just one user, until that user fixes their clock settings of course, but becomes a significant issue as soon as you need to coordinate timed actions between users.
Detecting the local time offset from your reference clock isn't always useful either: the machine's clock could be completely wrong so it says 01:23 where the user's wall-clock says 21:00 and your reference time matches neither.
Also some users may wish to fix your interface to a timezone other than the one currently set on the device they are currently using. Someone on holiday or on a business trip might want to keep some things linked to local time back home so they find it easier to coordinate with people there. Conversely someone currently in their home timezone might want to force your interface to match a non-local timezone for coordinating with teams/friends elsewhere in the world. You can't rely on them adjusting their clock in this circumstance as they may not be using their own machine and so might not be able to adjust the time/timezone settings (or may wish to keep them accurate to current local for other reasons).
tl;dr: Dealing with time values when there are any international concerns it harder than we tend to assume.