Having calibrated and adjusted many high precision DMMs (6+ digits), unfortunatelly the drift is not homogenous, i.e. you can e.g. have all except one within spec, and that one has drifted way beyond. If a meter was out of spec, that is what I saw most. Difting really depends on internal circuitry and so. Owning a precision DMM, and using it in a reasonable manner, also means calibration from time to time. Otherwise, what sense does it make to have one? Look for a cal lab that can do it at a reasonable cost. 150 euro with VAT or so including adjustment and report.