Entering and calculating hours for a 'timecard'

I'd like to calculate time for a timecard in Excel as a side exercise.
I'm currently using time in (D6) and time out (E6), both formatting HH:MM AM/PM with a total field reading =MOD(E6-D6,1)*24 formatted as General. This works well.

The problem is, you have to enter the time as "8:00 AM"
I've seen this replicated in an excel spreadsheet that allowed shorthand, for example "8 a" instead of "8:00 AM"
I can't figure out how to get data validation to allow users the option of entering their time in a shorthand format.

Any ideas?

In this case I woud be working with individuals that are very in their ways, the easiest solution is the accomodate them.

I am specifically looking for them to enter in 'time punches', using the example D6 and E6 from above. Some users would want to enter some users would use the format "8:00 am" and some would use "8 a". I'm interested in learning how to accomodate this, for my own understanding. I can't seem to figure it out!

I believe you when you say there is no easy way. I'm mostly motivated by my own desire to learn Excel better. I have seen this done before and am curious about how others would recreate it. Unfortunately that example was a protected file, so I can't dissect it.

It may also help to explain that I am allowing 'codes' to be entered into the initial punch field, as well as timestamps.
I've included the following picture with what I am trying to accomplish.
excel.jpg

Last call for any help, I've searched a lot of Google on this one.