extension requirements

[CALSQUID81]

I'm looking for a guideline (Z540, ISO 17025 etc.) that says whether or not you may extend the calibration of a standard after it has gone past its due date, and if so how long.

[RFCAL]

that is up to your quality manual

[beadwork]

You might want to do this only if you have enough objective historical evidence about the stability/accuracy of the standard.  This is used to support the decision to take the risk that extending the cal interval will result in an in-tolerance condition when the standard is re-calibrated.

Extending cal intervals is all about risk management.  What would be the ramifications if the standard was found to be out of tolerance at its next re-calibration?