For relative times (within the last week) there are phrases you can edit. The phrase names are in the page source:
Rich (BB code):
a_moment_ago: "A moment ago",
one_minute_ago: "1 minute ago",
x_minutes_ago: "%minutes% minutes ago",
today_at_x: "Today at %time%",
yesterday_at_x: "Yesterday at %time%",
day_x_at_time_y: "%day% at %time%",