What would happen if the International Date Line was defined as passing through Greenwich?
The International Date Line (IDL) is an imaginary longitudinal line from the North pole to the South pole approximately 180 degrees east (or west) around the globe from the Greenwich Meridian, which is also an imaginary longitudinal line from north to south. The IDL passes between the far east of Russia and the far west of the United States (Alaska) in an imaginary line down through the Pacific Ocean.
When traveling over the IDL from west to east you should adjust your watch exactly one day back, and when traveling over the IDL from east to west, adjust your watch exactly one day forward. This is because just over the other side eastwards of the IDL the day is 24 hours behind, and just over westwards the day is 24 hours ahead.
The Greenwich Meridian is designated as the World's Prime Meridian Line, with a longitude of zero degrees. All other longitudes are measured in degrees east or west of this line. It is also the astronomical zero-reference line from which astronomical bodies are located.
At midday in Greenwich it is midnight in places just east of the IDL on the day before the date that it is in Greenwich. On the other hand, in places just west of the IDL it is midnight on the same day that it is in Greenwich. Conversely, when it is midnight in Greenwich it is midday on the same day in places just east of the IDL but midday the next day in places just west of the IDL.
So, Greenwich is not on the International Date Line. It is on the Prime Meridian Line, i.e. zero degrees longitude. But if it were on the International Date Line, the roles would swap and what is now the IDL would be the Prime Meridian Line. Crossing the line at Greenwich eastwards would mean setting your watch back a whole day and crossing it westwards would mean setting your watch forwards a whole day.