Guidance around using google.type.Interval for timestamp ranges

aip/general/0145.md

@@ -48,6 +48,25 @@ Exclusive end values are preferable for the following reasons:
 - It is easier to reason about abutting ranges: `[0, x), [x, y), [y, z)`, where
   values are chainable from one range to the next.
 
+### Timestamp intervals
+
+The following section describes the use of the [google.type.Interval][interval]
+type, found amongst the common protos that are described in [AIP-213][].
+This type represents a range between two timestamps, with an inclusive start
+value and exclusive end value.
+
+Ranges between two timestamps which conform to the expectations of the
+`Interval` message **should** use this rather than having separate start and
+end fields. This allows client code to be written against the `Interval`
+message (such as checking whether a given timestamp occurs within the interval)
+and reused across multiple intervals in the same API, or even across multiple
+APIs.
+
+APIs **may** use start and end timestamp fields instead. In particular, if a
+message within an API is inherently describing an interval with extra
+information about that interval, the additional level of nesting introduced
+by using the `Interval` message may be undesirable.
+
 ### Exceptions
 
 In some cases, there is significant colloquial precedent for inclusive start
@@ -80,3 +99,6 @@ start and end values **should** use inclusive end values with `first_` and
 `last_` prefixes for those ranges only. The service **should** still use
 exclusive end values for other ranges where this does not apply, and **must**
 clearly document each range as inclusive or exclusive.
+
+[aip-213]: ./0213.md
+[interval]: https://github.com/googleapis/googleapis/blob/master/google/type/interval.proto