How to change a format of a custom field in MS Project 2010?

Question

I created some custom duration fields in Project 2010. They are all formula based and are based on the Work of the task; The work for my project is defined in terms of hours. However, the custom Duration fields (Duration1, Duration2, ..., Duration10) display in terms of days.

Is there any way to reformat those fields? Project 2010 seems to be missing a lot of functionality.

Thanks for any help, Bruno