Double Daylight Saving Time
Why was the sun rising as I walked to my Paris conference at 8:15 in the morning? I wondered. Thanks to Jim Lindgren, I now know the answer. In a "fall back" post on the Volokh Conspiracy, he explained:
It is pleasant traveling in France in the summer, which based on longitude should be on Greenwich time along with England, but instead is an hour ahead of London; the sun often goes down after 9:30pm. In effect, France and Spain are on double Daylight Saving Time in the summer and single Daylight Saving Time in the winter.
Peter Galison's book Einstein's Clocks, Poincare's Maps: Empires of Time details the intense Anglo-French rivalries over where to draw the time-establishing prime meridian.