Timesheet Calculations (have read but still can't figure out)

I have read many posts on here about how to get your timesheet (created in LiveCycle) to calculate hours but I am confused about what exactly to enter for the code.

Here's what I would like to happen:

RegHrs = ((LunchOutTime - StartTime) + (EndTime - LunchInTime)) / 100

This works great for the numeric fields which can only use military time to calculate correctly (hence the "divided by 100" to get a figure that looks like 8.0 hours). The problem is, no one wants to use military time.

What type of field do I need to use for regular time? Numeric? Date/time? Other?

And what code do I need to enter to get the results I need? For example,

Employee enters 8:00 am for StartTime, then 12:00 pm for LunchOutTime, then 1:00 pm for LunchInTime and then 5:00 pm for EndTime. And the form miraculously calculates 8 hours for them!

I really need this spelled out because I am extremely new to LiveCycle and haven't not had much training.

Thank you so much!