The international dateline roghly which meridian?Why does it vary from that meridian in places?

Answer 1

International dateline more or less follows the 180 degree (west) meridian.

It "goes around" islands and political unites so they can avoid being devided by data and time zone.

Answer 2

"For the most part, the International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. However, because the date to the east of the line is one day earlier than that to the west of the line, the line deviates to pass around the far east of Russia and various island groups in the Pacific, no country wanting to have, at least during ordinary daytime hours, its citizens functioning on two different dates. Thus, the two largest deviations from this meridian both occur to keep the date line from crossing nations internally."