A set of assertions about the provenance of things OPMGraph.]]> Observed Time allow for interval of observation, where an event is said to occur no earlier than a given time t1 and no later than a given time t2. When the event is observed to occur at a specific time, it is not convenient to use an interval. Instead, one can use the alternate exactlyAt attribute. We note that exactlyAt is disjoint from noEarlierThan and noLaterThan.