I've just tried it and I am afraid it's not possible. Right-click the taskbar, select Tool Setup. The digital clock is listed as stdclock.tool. But if you double-click it, there's no such an option. You can disable the digital clock, though.
A useful trick:
AmiStart taskbar is set to autosave any changes made to its configuration. And believe me, it's very easy to screw up something with AmiStart. Before you start experimenting, back up your current settings. It's the following file: