how do I add ddrop down field to a number

how do I add ddrop down field to a number

Hi, I am trying to use a drop down to select the hour for a scheduling app. I don't want to have 24 values because that is way too many so I want to use an am/pm radio button to create a calculation that will give me a 24 hour clock.

There is also a minutes drop down that is included in the calculation so that the final result should say 13:25 when the user puts 1 in the hour, 25 in the minute and selects PM for the radio selection

I have an if statement that captures for the am/pm not being checked, then a else if to set the time (initially in a display field but I will need to do this multiple times for different uses) then I have an else that should add 12 to the hour but instead of giving a result 13:25 it gives a result 112:25 in the Start_Time_Display single line field.

how do I get it to add the drop down value to the 12 - ie how do I turn the drop down value into a number not text

Thanks