Exactly. So if it works for Europe with New Zealand, it should work at least as well with Asia.
You should be weary with Spain, though. Even though it's quite out West, they're still using Central European Time (same as Hungary / Poland in the East), which is completely absurd. Hell, even France shouldn't be using CET, but WET / GMT. Only Portugal uses WET, and you have to go East to Finland / the Baltics / Romania / Bulgaria / Greece to switch time zones again (EET / GMT + 2). [0] is a handy map of time zones in Europe.
> The problem comes when you try to make THAT compatible with the California as well, which we determined it's just not possible in my last company.
You mean a call involving the three at the same time? Yeah, I can't see how that could be reasonable for all parties involved.
[0] https://en.wikipedia.org/wiki/File:Time_Zones_of_Europe.svg