A timestamp is an index-value pair. The index represents the point in the event processing chain at which the timestamp was recorded, for example "upstream entry to semantic mapper" and the value is a floating point number representing the time. The header file AP_TimestampSet.h defines a set of standard indexes, but a custom plug-in can define additional indexes for even finer-grained measurements. When you add a custom index definition, be sure to preserve the correct order, for example, an index denoting an "entry" point should be less than one denoting an "exit" point from that component.

Timestamps are relative measurements and are meant to be compared only to other timestamps in the same or similar processes on the same computer. Timestamps have no relationship to real-world "wall time".