Can a mind accommodate two time lines? Miles, Tan, Noble, Lumsden and Macrae (Psychonomic Bulletin & Review 18, 598-604, 2011) shows that Mandarin-English bilinguals have both a horizontal space-time mapping consistent with linguistic conventions within English and a vertical representation of time commensurate with Mandarin. However, the present study, via two experiments, demonstrates that Mandarin monolinguals possess two mental time lines, i.e., one horizontal and one vertical line. This study concludes that a Mandarin speaker has two mental time lines not because he/ she has acquired L2 English, but because there are both horizontal and vertical expressions in Mandarin spatiotemporal metaphors. Specifically, this study highlights the fact that a horizontal time line does exist in a Mandarin speaker's cognition, even if he/she is a Mandarin monolingual instead of a ME bilingual. Taken together, the evidence in hand is far from sufficient to support Miles et al.'s (2011) conclusion that ME bilinguals' horizontal concept of time is manipulated by English. Implications for theoretical issues concerning the language-thought relationship in general and the effect of bilingualism on cognition in particular are discussed.