Currently Roo does not configure a Dojo time picker for you (although there is one available). So if you choose a medium time format it will just expect the time in the correct format in your input field. Although the date picker actually does include the time it would have to be adjusted manually in the input field.
Code:
field date --fieldName test --type java.util.Date --timeFormat MEDIUM ...
So you would need to disable automatic scaffolding for the jspx in question and include the Dojo time picker in question.
Please feel free to open a Jira ticket for this!
Cheers,
Stefan