The meter is calibrated immediately before testing, with a 3 point curve - buffers 4, 7, and 10. Calibration is also verified with a second source standard, as well as a QC sample with a known value. I'd not trust a value unless it had been calibrated within 24 hours or so. That being said, the value would only be off probably by 0.1 or so... my home kit is based on a color comparison, which is more subjective.

Maybe my home kit has gone bad.