The different times for the British and American landings was down to a physical constraint: the tides. The Americans fixed the times for their landings at 06:30. This was an hour after low tide, when the water would be far down the beaches. As a result, the landing craft would avoid the German beach obstacles. Most of these had been placed so that they would damage landing craft that were landing at high tide. The Germans had made this assumption, as a landing at high tide would reduce the distance the troops had to cover on the beach, a wide-open target.

The Anglo-Canadian landings, to the east, had two different issues to face. Firstly, the timing of the tides were different. In the Channel, the tides move west-to-east. Low tide around Ouistreham, the eastern-most end of Sword Beach, is about 30-40 minutes after low tide on Utah Beach. This accounted for part of the delay. The rest was due to navigational hazards, especially around Sword Beach. Sword was sandwiched between the dangerous rocks of the Essarts de Bernières to the west and sandbanks around Merville to the east. The approaches to the other two beaches were also threatened by rocks. Fortunately for the Allies, most of these threats were only apparent at low tide. By delaying the British and Canadian landing, more time was gained for the water to cover them. As such, the landings were set at about 1 hour and 45 minutes after low tide. The landings on Juno Beach, where there was a particularly threatening reef, was delayed by ten minutes compared to the two British landings. The left flank of the Juno landing would receive a further ten minute delay as aerial reconnaissance had identified what appeared to be rocks near the surface at low tide. However, later examination would prove these to be seaweed - the delay had been unnecessary.

Interesting, thanks!

Is there any conjecture whether American losses at Omaha would not have been severe if they had landed at high tide? Maybe the losses suffered against landing obstacles to the Higgins would’ve been negated by the fact they lost so many troops crossing the open ground... This is fascinating! I’ve never seen attention drawn to the tides/landing times of D-Day...