What is the test for tolling a habeas petition?

When a petitioner asserts that a mental impairment was the cause of an untimely filing, equitable tolling is permitted if the following two factors are met: (1) the impairment was either so severe that the petitioner was unable to understand the need to timely file or his mental state rendered him unable to prepare his habeas petition and effectuate its filing, and (2) the impairment must have made it impossible to meet the filing deadline under the totality of the circumstances despite the petitioner's diligence. Bills v. Clark, 628 F.3d 1092, 1099-1100 (9th Cir. 2010).