What would be the time at Prime Meridian when a new day starts at 180° longitude?

When a new day starts at 180-degree longitude the time at the prime meridian will obviously be noon. It is 12 hours earlier when a new day starts at the International Date Line.

It's 00:00 on the Date Line. There are 24 hours in a day. On the prime meridian which is half the world away the time would be 24 divided by half which will come equally to 12.

Hence 12:00 would be the time on the Prime Meridian.