Expiry date of ISO 9001 Certificate

You should consider the correct expiry date the one expressed in your certificate. My reasoning is like yours. However, I checked several examples of certificates over the internet and I saw CB following your reasoning, and CB following your CB’s reasoning.

The following material will provide you with information about certification audits:

• ISO 9001 – How to know whether ISO 9001 certificate is valid? - https://advisera.com//9001academy/blog/2018/05/23/how-to-know-whether-iso-9001-certificate-is-valid/
• Surveillance visits vs. certification audits - https://advisera.com/27001academy/knowledgebase/surveillance-visits-vs-certification-audits/
• free online training ISO 9001:2015 Foundations Course – https://advisera.com/training/iso-9001-foundations-course/
• book - Discover ISO 9001:2015 Through Practical Examples - https://advisera.com/books/discover-iso-9001-2015-through-practical-examples/

If the date on calibration label have the same problem, which one is correct?

For example, calibration period is 1 year. Issue date: 20 Dec 2021. Expired date is 20 Dec 2022 or 19 Dec 2021?

I, as auditor, will check if the device is calibrated and within the calibration period on the calibration label. A calibration period of one year is not a scientific decision is an estimation. Following that reasoning we can start to differentiate leap years from other years to decide the calibration cut-off date. I, as auditor, want to know if there is a system in place and that it works, and that rules are complied with. It seems to me that it is not important to consider a year as 365, 366, or 367 days, but whatever the definition is, that it is followed.