protocol: Clarify timestamps

1 files changed, 7 insertions, 4 deletions

diff --git a/protocol.md b/protocol.md
index 549ac01..5951f11 100644
--- a/protocol.md
+++ b/protocol.md
@@ -17,6 +17,10 @@ Preliminary definitions:
 - A _word_ is a string without spaces (ASCII 32) (and also without newlines).
   Note that the empty string is also a valid word.
 
+Timestamps in the protocol, which indicate the time at which an event happened,
+are represented as microseconds since the UNIX epoch. A valid timestamp is
+always non-negative, i.e. at least 0.
+
 There are three kinds of messages: commands from the client, command responses
 from the server, and push messages from the server (not to be confused with push
 _notifications_). These three kinds are described below.
@@ -64,10 +68,9 @@ A _response_ from the server will be in one of the following forms:
 - `<tag> history_message <index:i64> <roomname:word> <user:word> <timestamp:i64> <msgid:i64> <replymsgid:i64> <message:string...>`
   - Part of the response to the client's `history` command. Index 0 is the
     oldest message; index (`<count>` - 1) (from the `history` response) is the
-    newest message in the fragment requested. Timestamps are microseconds since
-    the UNIX epoch. The reply message id is -1 if the message is a normal
-    message, or nonnegative if it is a reply to the referenced message in the
-    same room.
+    newest message in the fragment requested. The reply message id is -1 if the
+    message is a normal message, or nonnegative if it is a reply to the
+    referenced message in the same room.
 - `<tag> message <roomname:word> <user:word> <timestamp:i64> <msgid:i64> <replymsgid:i64> <message:string...>`
   - Response to the client's `get_message` command. This contains the
     information for a single message, as in `_push message` and the