How does this work if you get something like a W2, which is issued for the calendar year? Let's say your "personal" tax year starts and ends July 1 of each year

How does this work if you get something like a W2, which is issued for the calendar year? Let's say your "personal" tax year starts and ends July 1 of each year. Are you supposed to split your W2 in half and count each for each year? Sounds quite complex. I'm surprised they permit it.