If you mean before each message that is sent, then sorry but there isn't anything for that, but if you mean a timestamp simply in the conversation then a script has been released for that recently which puts the time zone in the conversation.
I know this isn't related to that, but I'm sure you could easily modify his code to have it so it's a timestamp with the date etc. in there.
[Release] Timezone v1.01.020
Other than that, no sorry.