To shorten or lengthen a time block you'll need to find and delete one or both in and out transactions, delete them, and then add a new one/s with the correct DateTime.
Step 1: Click on the time block from the timesheet view
Step 2: Locate the in or out transaction that needs to be modified
Step 3: Delete the erroneous in or out transaction
Step 4: Click New In/Out to add a new transaction for the correct date and time.