Timing Ruler   A timing ruler is a graduated device for indicating the passage of time on a timing diagram. The graduations are called ticks and are typically placed on the lower edge of the diagram frame. Time runs from left to right. The scale and time units (ticks) depend upon what the diagram represents and may be sub-second units such as milliseconds or larger time divisions such as hours, days, months or even years.

Explanation

Timing diagrams are used to show a time-oriented view of an interaction. The use of a timing ruler allows a modeler to definitively describe when events occur and when states and values change on one or more lifelines. Timing rulers are typically used to show relative time but can also be used to show absolute time when events must occur at a given real world time or date.