Certain English 'non-root modal + PERF' combinations are ambiguous between epistemic and metaphysical readings. The ambiguity has been traditionally analyzed as a scopal one. In this paper the author proposes an alternative temporal shifting analysis which avoids the problems with the competing scopal analysis. The metaphysical reading results when the evaluation time of the non-root modal is backward shifted to its topic time by an operator, and the epistemic reading does not involve temporal shifting. The postulation of the temporal shifters finds crosslinguistic evidence in Mandarin modal contexts.