InfoPath Dev

In Sharepoint I have a list created. In the list I have times in a drop-down list using this fomat: 8:30 AM

I have seen posts without the AM/PM specification; however, I really want to use it.

I use this calculation and it works beautifully in the Sharepoint list to calculate the amount of time between two times.  Where Sun1Out = Stop Time and Sun1in = Start Time.

=((SunOut1-INT(SunOut1))*24)-((SunIn1-INT(SunIn1))*24)

 I am using Infopath 2010 to create my form but it will not take this calculation.  How should my formula be written in Infopath or what functions should I use?

Thank you

 If the calculation is already done in the SharePoint list, you could create a form load rule or a query for data rule to set the 'total' field as required.