bug report: tf with negative time

I would like to report a bug in time display with tf(event,hh:mm) there the event is in the past.

I wanted to use this to display the time since last missed call; and the 'x minutes ago' notation takes up too much space so I prefer the hh:mm notation. But it will display something like 00:-24

It works ok for positive times, but limps for negative times.