What is the legal test for equitable tolling in a habeas petition?

³ Applying this standard will require the district court to evaluate how a petitioner's mental impairment bore on his ability to file. The relevant question is: Did the mental impairment cause an untimely filing? See Spitsyn, 345 F.3d at 799 (equitable tolling is available if it was "impossible to file a petition on time"); but see Harris v. Carter, 515 F.3d 1051, 1055 n. 5 (9th Cir.2008) ("Despite the unequivocal 'impossibility' language in our standard, we have not insisted that it be literally impossible for a petitioner to file a federal habeas petition on time as a condition of granting equitable tolling. We have granted equitable tolling in circumstances where it would have technically been possible for a prisoner to file a petition, but a prisoner would have likely been unable to do so.").