Finding the total number of hours logged in

There is this google form where people would enter the time they login and then enter the time they logout from - let's say volunteering. The point of the google sheet is to calculate the total number of hours a certain person volunteered for the entire time.

Ex. If I chose to see how many hours "John" volunteered for, it should show the total hours he has done based on the data he entered

I have also included the linked to a copy of the google sheets if my explanation doesn't really make sense. LINK In the next sheet, I have created a timesheet for 5 days of the week that the user choose. In other words, it calculates the total hours for 5 days that the table shows.

I researched how to separate the data where if the name is "John", then only look at the data given for John and no one else...then calculating the total hours, but none gave a specific answer for an if statement looking at a column for a certain word.

I want to somehow make the sheet to ignore the dates and, for example, if I choose "John", it will only calculate the hours for each time he clocks in and displays the total amount of hours he volunteered. Any tips would be helpful!