Calculating time difference with different formats

I would like to calculate a difference in seconds between two columns of timestamps
1. First one is "00:34:52" and formatted as "00:34:52" in Custom hh:mm:ss format.
2. Second is "12:34:55 AM" and formatted as "00:34:55" in Custom hh:mm:ss format.

I tried the recommended =TEXT(B2-A2,"ss") with different permutations in the formatting but to no avail.

Any suggestions are deeply appreciated.