I've not got IP or SP to hand. Yet in excel you can work out what you want as follows using the following formula which you could try to see if it would work in a calculated column.
B2 is your End Date and Time, A2 is your start Date and Time.
=INT(B2-A2)&" days "&TEXT(B2-A2,"h"" hrs ""m"" mins ""s""secs""")
It assume that your original time in "A/B 2s" are in the following format: dd/mm/yyyy hh:mm:ss
If not crack open Excel and work with the formula and others. If you can do it in excel you can normally find a similar way to do it in InfoPath if used with SharePoint.