Time is conceived as being linear and unidimensional, so the concept of measure is important to time. Temporal measure is seen as duration. On the linear time scale, if we link duration to a specific position, we have the concept of span. Like spatial direction, span can be divided into forward span, expressed by the prepositions ‘dào/zhídao’ (up to (a specified point in time)) and backward span, expressed by the prepositions ‘zìcóng’ (since) and ‘cóng…yǐlái…’ (from…), etc. Forward span extends from the speaker’s ‘now’ forward to some point that is future in relation to the ‘now’ (‘now’ refers to the point in time to which the speaker is oriented, and of primary concern to the speaker within a given context). The reverse of the forward span is the backward span, which indicates a stretch back in time from the speaker’s ‘now’ (Quirk, et al., 1985)
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.