Longitude.
The vertical one that goes from negative to positive at 180 Longitude East or West.
Maybe you should get a map out and have a look.
I was working in Samoa when they changed the date to align with NZ time, so am quite familiar with the region.
I mainly intended to clarify for any readers rather than castigate you, and after the initial remark about time zone I got a little blunt.
( FWIW I made several mistakes in this comment and had to edit twice )
Longitude.
The vertical one that goes from negative to positive at 180 Longitude East or West.
Maybe you should get a map out and have a look.