diff --git a/irc/client.go b/irc/client.go
index f28153db..3d920910 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -580,14 +580,14 @@ func (session *Session) playResume() {
 
 	timestamp := session.resumeDetails.Timestamp
 	gap := lastDiscarded.Sub(timestamp)
-	session.resumeDetails.HistoryIncomplete = gap > 0
+	session.resumeDetails.HistoryIncomplete = gap > 0 || timestamp.IsZero()
 	gapSeconds := int(gap.Seconds()) + 1 // round up to avoid confusion
 
 	details := client.Details()
 	oldNickmask := details.nickMask
 	client.SetRawHostname(session.rawHostname)
 	hostname := client.Hostname() // may be a vhost
-	timestampString := session.resumeDetails.Timestamp.Format(IRCv3TimestampFormat)
+	timestampString := timestamp.Format(IRCv3TimestampFormat)
 
 	// send quit/resume messages to friends
 	for friend := range friends {
@@ -596,23 +596,29 @@ func (session *Session) playResume() {
 		}
 		for _, fSession := range friend.Sessions() {
 			if fSession.capabilities.Has(caps.Resume) {
-				if session.resumeDetails.HistoryIncomplete {
+				if !session.resumeDetails.HistoryIncomplete {
+					fSession.Send(nil, oldNickmask, "RESUMED", hostname, "ok")
+				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
 					fSession.Send(nil, oldNickmask, "RESUMED", hostname, timestampString)
 				} else {
 					fSession.Send(nil, oldNickmask, "RESUMED", hostname)
 				}
 			} else {
-				if session.resumeDetails.HistoryIncomplete {
-					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected (up to %d seconds of history lost)"), gapSeconds))
-				} else {
+				if !session.resumeDetails.HistoryIncomplete {
 					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected")))
+				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
+					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected (up to %d seconds of message history lost)"), gapSeconds))
+				} else {
+					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected (message history may have been lost)")))
 				}
 			}
 		}
 	}
 
-	if session.resumeDetails.HistoryIncomplete {
+	if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
 		session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", fmt.Sprintf(client.t("Resume may have lost up to %d seconds of history"), gapSeconds))
+	} else {
+		session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Resume may have lost some message history"))
 	}
 
 	session.Send(nil, client.server.name, "RESUME", "SUCCESS", details.nick)
diff --git a/irc/commands.go b/irc/commands.go
index 2b221621..f5049948 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -232,7 +232,7 @@ func init() {
 		"RESUME": {
 			handler:      resumeHandler,
 			usablePreReg: true,
-			minParams:    2,
+			minParams:    1,
 		},
 		"SAJOIN": {
 			handler:   sajoinHandler,
diff --git a/irc/handlers.go b/irc/handlers.go
index 98b77c8d..e6cf61ce 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -2359,24 +2359,27 @@ func renameHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Re
 	return false
 }
 
-// RESUME <token> <timestamp>
+// RESUME <token> [timestamp]
 func resumeHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *ResponseBuffer) bool {
+	details := ResumeDetails{
+		PresentedToken: msg.Params[0],
+	}
+
 	if client.registered {
 		rb.Add(nil, server.name, "FAIL", "RESUME", "REGISTRATION_IS_COMPLETED", client.t("Cannot resume connection, connection registration has already been completed"))
 		return false
 	}
 
-	ts, err := time.Parse(IRCv3TimestampFormat, msg.Params[1])
-	if err != nil {
-		rb.Add(nil, server.name, "FAIL", "RESUME", "INVALID_TIMESTAMP", client.t("Cannot resume connection, timestamp is not valid"))
-		return false
-	}
-
-	rb.session.resumeDetails = &ResumeDetails{
-		PresentedToken: msg.Params[0],
-		Timestamp:      ts,
+	if 1 < len(msg.Params) {
+		ts, err := time.Parse(IRCv3TimestampFormat, msg.Params[1])
+		if err == nil {
+			details.Timestamp = ts
+		} else {
+			rb.Add(nil, server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Timestamp is not in 2006-01-02T15:04:05.999Z format, ignoring it"))
+		}
 	}
 
+	rb.session.resumeDetails = &details
 	return false
 }