rpc: clarify timestamps (#304)

* clarify timestamps

* changelog entry

* Update spec/rpc/README.md

Co-authored-by: Callum Waters <cmwaters19@gmail.com>

Co-authored-by: Callum Waters <cmwaters19@gmail.com>

CHANGELOG.md

@@ -5,3 +5,4 @@ All notable changes to the spec will be documented in this file for easy referen
 ## [Unreleased]
 
 - [RPC] [#276](https://github.com/tendermint/spec/pull/276): Add specification for RPC (@marbar3778)
+- [RPC] [#304](https://github.com/tendermint/spec/pull/304): Clarify timestamp format (@marbar3778)

spec/rpc/README.md

@@ -41,6 +41,14 @@ This file defines the JSON-RPC spec of Tendermint. This is meant to be implement
   | [BroadcastTxAsync](#broadcasttxasync)   |                             ✅                              |                                 ✅                                 |
   | [BroadcastEvidence](#broadcastevidence) |                             ✅                              |                                 ✅                                 |
 
+## Timestamps
+
+Timestamps in the RPC layer of Tendermint follows RFC3339Nano.  The RFC3339Nano format removes trailing zeros from the seconds field.
+
+This means if a block has a timestamp like: `1985-04-12T23:20:50.5200000Z`, the value returned in the RPC will be `1985-04-12T23:20:50.52Z`.
+
+
+
 ## Info Routes
 
 ### Health