Vim script: how to retrieve the timezone offset?

I would like to get the timezone offset of the user environment in a Vim script and that must be portable (Unix/linux/MacOSX, Windows).

For example, if the user is in Paris in summer, 开发者_JAVA技巧its timezone offset is currently +2 (UTC+2).

Portability concerns exclude using external programs or non native scripting engines (Perl, Python, Ruby...).

:echo strftime("%z") is as portable as the %z format specifier is.