Fresh air calibrating a meter is not a substitute for actualling getting all sensors done with the proper calibration gas every 6 months.
It's just a means of self/test calibrating and mainly for the o2 sensor anyway.
A meter more than a month outside calibration isn't safe, I know this as I always take 2 out down in the sewers as there are usually at least 2 of us.
This one time in London the 'out of calibration' meter was happy enough although the o2 meter was a bit jumpy, but we hit some h2s and methan that wasn't safe to traverse any further in as the recently calibrated meter alerted us to that fact!
I don't mess about with my 3 anymore, they get done every 6 months no matter what, I may need to sell one soon as I don't really have a use for 3 these day's..
Safety is paramount