The pillar, cylindrical or shepherd's sundial is very similar to the one represented on the shelf in the top left corner of Holbein's portrait of Nicholas Kratzer:

The dial in The Ambassadors has been shown to be set for April 11 or August 15. Although, as noted by Foister,* there is no certainty that de Selve was in London as early as April 11, this earlier date seems likely since April 11 was no ordinary date, but rather was Good Friday in 1533. This should be connected to other details in the painting that point to Christ and His act of redemption through his sacrifice: half-hidden crucifix and the Lutheran Hymnbook.