BuddyPress.org

Thinking the issue lies with:

/messages/messages-loop.php

bp_message_thread_last_post_date()

function lives in:
/buddypress/bp-messages/bp-messages-templatetags.php: line 288

would guess at the filter

bp_format_time(get_date_from_gmt(

is possibly to blame but will have to see if the ticket sheds light on it