Trip Time / Total Time

I use a Montana 6xx when I fly hot air balloons.  I like to have a timer (which I thought used to be called trip time) to keep track of how long I have been in the air.  Right before taking off I turn my Montana on.  This used to give me a close to accurate time of how long I have been in the air since turning the GPS on.  It seems now that it takes into account the last time I reset the trip date (probably after my last flight).  Because of this, the timer isn't accurate for keeping track of how long I have been in the air   Is there a way or am I using the wrong parameter to achieve this.