consolidate some time-setting code

irc/client.go

 		msg.SetTag("msgid", msgid)
 	}
 	// attach server-time
-	if session.capabilities.Has(caps.ServerTime) {
-		if serverTime.IsZero() {
-			serverTime = time.Now().UTC()
-		}
-		msg.SetTag("time", serverTime.Format(IRCv3TimestampFormat))
-	}
+	session.setTimeTag(&msg, serverTime)
 
 	return session.SendRawMessage(msg, blocking)
 }
 
 func (session *Session) Send(tags map[string]string, prefix string, command string, params ...string) (err error) {
 	msg := ircmsg.MakeMessage(tags, prefix, command, params...)
+	session.setTimeTag(&msg, time.Time{})
+	return session.SendRawMessage(msg, false)
+}
+
+func (session *Session) setTimeTag(msg *ircmsg.IrcMessage, serverTime time.Time) {
 	if session.capabilities.Has(caps.ServerTime) && !msg.HasTag("time") {
-		msg.SetTag("time", time.Now().UTC().Format(IRCv3TimestampFormat))
+		if serverTime.IsZero() {
+			serverTime = time.Now()
+		}
+		msg.SetTag("time", serverTime.UTC().Format(IRCv3TimestampFormat))
 	}
-	return session.SendRawMessage(msg, false)
 }
 
 // Notice sends the client a notice from the server.

irc/responsebuffer.go

 		msg.SetTag("msgid", msgid)
 	}
 	// attach server-time
-	if rb.session.capabilities.Has(caps.ServerTime) && !msg.HasTag("time") {
-		msg.SetTag("time", time.UTC().Format(IRCv3TimestampFormat))
-	}
+	rb.session.setTimeTag(&msg, time)
 
 	rb.AddMessage(msg)
 }
 		// ACK message
 		message := ircmsg.MakeMessage(nil, rb.session.client.server.name, "ACK")
 		message.SetTag(caps.LabelTagName, rb.Label)
-		if rb.session.capabilities.Has(caps.ServerTime) {
-			message.SetTag("time", time.Now().UTC().Format(IRCv3TimestampFormat))
-		}
+		rb.session.setTimeTag(&message, time.Time{})
 		rb.session.SendRawMessage(message, blocking)
 	} else if useLabel && len(rb.messages) == 1 && rb.batchID == "" && final {
 		// single labeled message
 	// send each message out
 	for _, message := range rb.messages {
 		// attach server-time if needed
-		if rb.session.capabilities.Has(caps.ServerTime) && !message.HasTag("time") {
-			message.SetTag("time", time.Now().UTC().Format(IRCv3TimestampFormat))
-		}
+		rb.session.setTimeTag(&message, time.Time{})
 
 		// attach batch ID, unless this message was part of a nested batch and is
 		// already tagged