Javascript: Getting invalid Time Zone

[Waqas_Badar]

I have found following piece of code for client time zone:

alert((new Date().getTimezoneOffset()/60) * -1)

It works fine for positive time zone. But when set negative time zone on system. I got one hour later e.g. if i set -8 time zone on system then i got -7.
ANY IDEA HOW CAN I RESOLVE IT?