Is synaesthesia cognitively useful? Individuals with time-space synaesthesia experience time units (such as months of the year) as idiosyncratic spatial forms, and report that these forms aid them in mentally organising their time. In the present study, we hypothesised that time-space synaesthesia would facilitate performance on a time-related cognitive task. Synaesthetes were not specifically recruited for participation; instead, likelihood of time-space synaesthesia was assessed on a continuous scale based on participants' responses during a semi-structured interview. Participants performed a month-manipulation task, which involved naming every second month or every third month in reverse-chronological order, beginning and ending with a target month. Using hierarchical multiple regression, we found that time-space synaesthesia corresponded with faster performance on both versions of the task. We propose that time-space synaesthesia may expedite the cognitive manipulation of time-based information. Our results also indicate that synaesthesia is far less unusual than widely believed.