How to calculate time

Hi! I'm new here so please pardon me if this should go in another thread and not in a new one.
So here's my problem. I have a list of employees with login and logout time. An employee may have more than 1 login time. If a login is found, check if there's a corresponding logout time. If none, return blank, if yes, check for another login. If same condition is found, continue checking until a login with no logout time is found then return blank. If no other login is found, calculate the time and if time is less than 9 hours, return the value, else return blank. Whew, I hope I made it clear.

I have attached an example.

Looking forward and thanks in advance.

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