Many database Management systems and operating systems provide support for time values. At the physical level time values are known as timestamps. A timestamp has a physical realization and a temporal interpretation. The physical realization is a pattern of bits while the temporal interpretation is the meaning of each bit pattern, that is, the time each pattern represents. All previous proposals defined timestamps in terms of seconds. However, as we show, there are at least seven definitions of this fundamental time unit. We propose a more precise temporal interpretation, the time-line clock, that constructs a time-line by using different well-defined clocks in different periods. We also propose timestamp formats for events, intervals and spans. These formats can represent all of time to the granularity of a second, and all of recorded history to a finger of granularity of a microsecond. Our proposed formats were designed to be more space and time efficient than existing representations. We compare our formats with those used in common operating systems and database management systems.