Use the parameters under Display to control the display of field groups. Specifically the "Custom Display Rows" parameter. It even supports template based output and custom templating if you'd prefer to style the display using a template. Both profile display and edit display can be changed in that tab. Please be sure to read parameter descriptions carefully as it explains how to properly substitute in those fields.

If you're trying to substitute those fields outside of the field group it gets a bit more tricky, but is doable. See the below on how to do this.

www.joomlapolis.com/forum/153-professional-member-support/242183-accessing-the-individual-fields-within-a-field-group#316989

Ah, thought you wanted to just display them side by side. Sorry, misunderstood what you're asking. Believe only way you can do that is use a code field inside your field groups field then handle the math operations there. The substitutions for those fields values (e.g. [cb_startdate]) should work fine as long as the code field is inside of the field group field. You'll need to write the necessary PHP to handle your math operations though.