Clarify event timestamp origin and range

CHANGELOG.md

@@ -22,6 +22,7 @@ Updates:
   * SpanProcessors must provide read/write access at least in OnStart.
 - Specify how `Probability` sampler is used with `ParentOrElse` sampler.
 - Clean up api-propagators.md, by extending documentation and removing redundant sections ([#577](https://github.com/open-telemetry/opentelemetry-specification/pull/577))
+- Document that event timestamps might be out of range ([#839](https://github.com/open-telemetry/opentelemetry-specification/pull/839))
 
 ## v0.6.0 (07-01-2020)
 

specification/trace/api.md

@@ -448,6 +448,12 @@ The Span interface MUST provide:
 Events SHOULD preserve the order in which they're set. This will typically match
 the ordering of the events' timestamps.
 
+Consumers should be aware that an event's timestamp might be before the start or
+after the end of the span if such timestamp was provided by the user to the
+`AddEvent` API.
+The specification does not require any normalization if the provided timestamp
+is out of this range.
+
 Note that the OpenTelemetry project documents certain ["standard event names and
 keys"](semantic_conventions/README.md) which have prescribed semantic meanings.