Here is a freemarker REST call you could make to get the message post time:
<#assign post_time = rest("/message/id/${message_id}/post_time").value />
Then you can use freemarker to format the date, or if you have friendly dates turned on you could do something like this:
<#assign post_friendly_date = post_time.@view_friendly_date />
Here are some relevant freemarker articles:
http://freemarker.sourceforge.net/docs/ref_builtins_date.html
http://freemarker.sourceforge.net/docs/xgui_imperative_learn.html#autoid_70
-Doug