SAN FRANCISCO (CBS SF) — The average minute will briefly gain an extra second Tuesday, to make up for the earth’s slowing rotation, and stock exchanges around the globe are bracing for it.

Clocks went atomic in 1967, but since then the pace of the earth’s rotation has slowed.  To keep clocks in sync with the genuine time, scientists have added a “leap-second” onto the calendar every few years.  Until now, they have been able to avoid having the change happen during active trading.

But, June 30th will be the first leap second to happen during trading hours, Bloomberg reported.  The change is scheduled for 8 p.m. eastern time, just as the markets in Asia open.

The fastest trading firms have computers that can react to market changes in milliseconds, so exchanges around the world are taking steps from closing trading early, to delaying all market transactions.