Currently Spark is not showing a time stamp for user join/leave messages, as well for topic changes and messages with /me command. Maybe also for other messages when an admin grants a voice, bans a user, etc. It could be showing the same time stamp as for the common messages, e.g.:
(13:20) John has joined the room (green text)
(13:20) John: Hi! What a nice day!
(13:21) John changed the subject to "What a nice day!" (green text)
(13:22) John is sleepy (pink text)
(13:23) John has left the room (green text)