what was the largest difference between GMT and UTC in the current epoch?”
Basicly the difference comes down to GMT being based on the rotation of the earth around its axis and the sun (which isn’t completely regular) and UTC being based on a Cesium atomic clock (which is far more accurate and regular). UTC is regularly modified with “leap seconds” so that it matches up to GMT to be the standard for date/time stamps. Also, UTC is the authoriative measurement for calculations involving duration.
