Here's how I understood GM's reasoning when it announced the big loss last year:
General Motors wrote off the books several billion dollars. The accounting actually occurred last year. The dollars written off did not exist as you and I understand "dollars." The accounting entries consisted of the asset value of tax deductions which GM had carried forward from previous years in which they had insufficient profits to use those deductions in full.
Why did they write them off? An accountant could answer that better than I, however, the stated reason was to conform the books to a more realistic statement of GM's true asset level. They were acknowledging that the carry forward tax deductions would expire before GM would make enough money to use them.
Cheers,
Ed