time formats and time duration formula

I am creating a spreadsheet to track my time and hours worked. I have my Start Time and my End Time in h:mm AM/PM format, I then have a column for total hours worked in hh:mm format. Something like this: Column A 9:00 AM, column B 4:30 PM, and my total hours worked column = 7:30 (meaning 7.5 hours worked) my formula there is =B1-A1. I am getting the duration of time lapsed between 9am and 4:30pm. That's fine.
My question is, I want my column B to default to PM so I don't have to type it. When I change the format to show PM, it puts it in military time. I want standard time. So, I did a custom format of h:mm "PM". That didn't work. It turned my "total hours worked" column into this ############### value. Can I have both, standard time and my duration of time formula?

Attach a sample workbook. Make sure there is just enough data to demonstrate your need. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are shown, mock them up manually if necessary.

Remember to desensitize the data.

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Time formats are notoriously difficult, at least for me, to deal with. You may have to deal with typing in AM/PM if you don't want to deal with a 24-hour time format. I don't know if there is a way for excel to automatically distinguish between 9:00AM and 9:00PM without the AM/PM, at least not without vba. I'm playing with it, will see what I can come up with.

Originally Posted by protonLeah

Do not type in your start & end times. Use Ctrl+shift+: in the appropiate cell.

This is a very good point. CTRL+: enters the current time. If you use this then you don't have to type in a time at all, and you don't need my code.

I tried your formula and it works great 6StringJazzer. But what is the actual code you have in the End Time cell?

If you're wanting to just always assume B is PM
Go ahead and just type it as an AM time (same as A), then use this formula to calculate the duration
=(B1+0.5)-A1