The controls in the Default Clock pane of World Clock Deluxe Settings (macOS 13 or later) or World Clock Deluxe Preferences (macOS 12 or earlier) don't affect existing clocks: World Clock Deluxe uses the default clock format just to set up new clocks.
Choose Pane > Clocks.
Select the clocks and choose Batch Change from the Action pop-up menu at the bottom of the list.
To select clocks that are next to each other (contiguous) in the list, press the Shift key and then click the first and last clock; to select clocks that are not next to each other in the list, press the Command key and then click each clock.
Specify a format.
Click OK.
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