How do I change the clock format in rox-icewm?

I've just installed an antiX system with rox-icewm as the window manager. I want to change the format of the displayed date/time on the taskbar. According to the manual I need to edit the ~/.icewm/preferences file. So I did that and saved it, remembering to remove the comment characters.

But the format remains stubbornly with a US-style date. This has survived several reboots.

Any ideas what I'm doing wrong (I'm a Linux noob) please?

Peter