I did a couple of years flying HM's finest plastic motor gliders and indeed used to log that as JW411 said basis time in the air only. Back in the day you could also log that as Group A so it also used to go in my Civvie logbook with 5mins added either side for taxi time, thnking back thats probably where I got the 5mins either side habbit from.
That and rounding everything up / down to the nearest 5mins on the Form 700 since we werent the most mathematically talented bunch...