Hoerl & McCormack (H&M) distinguish temporal updating and temporal reasoning systems, but do not elaborate on how the temporal reasoning system develops. This commentary considers the extent to which the temporal reasoning system depends on language and other external representations in its development, while aiming to answer H&M's question of how to characterize temporal cognition in early childhood. As with numerical cognition (Núñez Reference Núñez2017), there appears to be a symbolic reference gap between a non-symbolic (updating) mode of information processing accessible to infants and a symbolic (reasoning) system that develops through explicit teaching and enculturation. We argue that this gap is overcome in childhood through piecemeal acquisition of verbal and visual-spatial means of explicitly representing and organizing notions of time.

Children's acquisition of language is inextricably linked with their expanding capacities to make sense of objects and events in the world. Words draw attention to things and their recurrent use invites children to search for underlying commonalities across referents and situations (Plunkett et al. Reference Plunkett, Hu and Cohen2008; Waxman & Markow Reference Waxman and Markow1995). For over 70 years, developmentalists have traced the emergence of temporal language through children's acquisition of grammatical aspect, tense marking, deictic expressions (e.g., now, then), and labeling of temporal patterns (e.g., days of the week) (Ames Reference Ames1946; Wagner Reference Wagner2001; Weist Reference Weist, Levin and Zakay1989). As with other words, children are exposed to temporal terminology informally in social contexts and familiar routines. Such terms distinguish time based on arbitrary divisions (e.g., minutes, hours), natural cycles (e.g., day, night), social events (e.g., lunchtime, Thanksgiving), or sequences (e.g., before, after). Through conversational discourse, children gain familiarity with the corresponding concepts, although they may use terms in ways that do not fully reflect conventional meanings (Levy & Nelson Reference Levy and Nelson1994). For example, preschoolers may interchangeably use yesterday and tomorrow to refer to times not today (Harner Reference Harner1975), indicating that their production of temporal vocabulary often precedes accurate comprehension.

Nelson (Reference Nelson1985; Reference Nelson1996) proposed a dialectical model of language development in which children derive the meaning of words from their patterns of use in discourse contexts. She extended the model to children's acquisition of temporal concepts, theory of mind, and autobiographical memory, proposing that through social transactions with caregivers, children acquire conventional ways of structuring knowledge and gain flexibility in adopting varying and potentially conflicting perspectives on events (Nelson Reference Nelson, Gelman and Byrnes1991; Nelson & Fivush Reference Nelson and Fivush2004; Nelson et al. Reference Nelson, Skwerer, Goldman, Henseler, Presler and Walkenfeld2003). Caregivers’ use of evaluative language, often delivered in the context of narratives, supplements children's script-like representations of events by imposing a causal-intentional framework for making sense of human behavior. As described by Tomasello (Reference Tomasello1999, p. 214), “Language is structured to symbolize in various complex ways events and their participants, and this is instrumental in leading children to ‘slice and dice’ their experience of events in many complex ways.” Caregivers scaffold children's acquisition of temporal concepts by engaging children in conversations littered with sequence terms, conventional time markers, and references to the immediate past and future plans (Hudson Reference Hudson2002; Reference Hudson2006). Such markers help children track the activities of people (including themselves) over time, and therefore contribute to the emergence of a temporally extended self-concept.

Karmiloff-Smith (Reference Karmiloff-Smith1979; Reference Karmiloff-Smith1992) similarly viewed conceptual development as embedded in interactive contexts, while emphasizing its intrinsic connection with the child's sensorimotor activity. In her theory of representational redescription, she aimed to account for the process by which the child's early proceduralized knowledge is redescribed into more explicit, abstract, and conventionalized formats. According to Karmiloff-Smith (Reference Karmiloff-Smith1992), it is not language per se that is responsible for developmental change, but the redescriptive processes that allow implicitly represented knowledge to be flexibly re-represented in multiple formats (including language) that are increasingly accessible to consciousness and meta-cognitive reflection. Such symbolic formats include image schemas (Mandler Reference Mandler2004), generalized event representations (Nelson & Gruendel Reference Nelson, Gruendel, Lamb and Brown1981), gestures (Goldin-Meadow Reference Goldin-Meadow2003), drawings (Goodnow Reference Goodnow1977), and other forms of visual-spatial notation that may be intermediary to the fully explicit representations assumed by H&M to underlie the temporal reasoning system.

In H&M's account, temporal reasoning requires the construction of a linear representation of time, where each time occupies a unique, unrepeated location. Núñez (Reference Núñez2011) summarizes the culturally and historically mediated processes that led to the invention of the number line concept, with its left-to-right mapping of quantities onto a spatial representation where each number has a discrete location. Acquiring the number line concept would provide the child with an organizational framework for constructing a mental time line (Núñez & Cooperrider Reference Núñez and Cooperrider2013). However, as noted by H&M, situating events in time is complicated by the ever-shifting present and our capacity to treat any moment as the “present” in a given discourse or narrative context. Hence, flexible perspective taking, which develops alongside theory of mind, may prove critical for shifting one's viewpoint away from the immediate here and now. H&M and others note considerable overlap in the frames of reference used in positioning objects and events across temporal and spatial domains, while acknowledging that representations vary considerably as a function of sociocultural and linguistic factors (Casasanto & Boroditsky Reference Casasanto and Boroditsky2008; Tenbrink Reference Tenbrink2007). As described by Núñez and Cooperrider (Reference Núñez and Cooperrider2013, p. 220), “Time is not a monolith, but rather a mosaic of construals with distinct properties and origins.” Similar to language (Tamariz & Kirby Reference Tamariz and Kirby2016), temporal representations may become conventionalized through processes of cultural transmission, and therefore may not be universal.

H&M note the substantial challenge of accounting for the developmental emergence of the temporal reasoning system. We believe that acknowledging the co-existence and diversity of representational formats lessens this concern. The child's ability to reason about time capitalizes on available representational formats, which include the clock and calendar systems formally taught in school (Burny et al. Reference Burny, Valcke and Desoete2009). Such formats represent time in publically accessible ways, and therefore allow it to become an object of contemplation and negotiation. Although details remain to be worked out, especially with regards to individual differences in developmental trajectories, there is growing consensus that language and other symbolic reference systems mediate cognitive development, ultimately serving to extend cognition by offloading processing to external media (Clark & Chalmers Reference Clark and Chalmers1998).