The term ``time-line'' has a well-understood informal meaning, as does ``clock,'' which we coopt for this definition (+E5). This concept currently has no name (+E7)(-E3), but it is used for every timestamp (e.g., SQL2 uses the mean solar day clock—the basis of the Gregorian calendar—as its time-line clock). The modifier ``time-line'' distinguishes this clock from other kinds of clocks (+E1). Time-line is more intuitive than ``base-line'' (+E8), but less precise (mathematically) than ``time-segment,'' since the time-line clock usually describes a segment rather than a line (-E9). We prefer time-line clock to time-segment clock because the former term is more general (+E4) and is intuitively appealing.