You can dismiss the warning that appears by pressing the centre button, and the instrument will function as normal. This message depends on the calibration date programmed into the instrument, which will match the calibration date declared on the certificate. The calibration is valid to 1 year from the date of calibration, or from the date of first use (if this has been filled in). After this, the instrument can be used for comparative use only. It cannot be used as part of a traceable process.