I used mercury-type sphygmomanometers in hospital practice for many years. These were never recalibrated. Latterly, I began to doubt the accuracy of my readings, since these depended, in part, on the acuity of my hearing, and blessed the day when electronics came to the rescue.
The best home monitors are those which use an upper-arm cuff, rather than a wrist-cuff. Of the 2 readings the diastolic (lower) is the more significant, as this is the residual pressure in the arteries between heart beats. Even minor exercise will tend to make the systolic pressure vary, as will one's emotional status, hence the 'white-coat phenomenon'.
I trust my home monitoring, taken weekly during a period of absolute rest, and doubt the accuracy of readings. taken at GPs surgeries.