Question

what would happened if the International Date Line passed through Greenwich

Answer 1

No, the International Date Line does not pass through Greenwich, England. The International Date Line is only in the Pacific Ocean, on more or less the exact opposite side of the world from Greenwich.

The International Date Line is (more or less) the continuation (or the opposite side) of the Prime Meridian. The Prime Meridian does pass through Greenwich. However, the Date Line is confined to the other side of the world. In other words, the Prime Meridian is at 0 degrees of longitude while (most of) the International Date Line is at 180 degrees.