The code does not show how the expiry value in both cases is being calculated. From what is said it look like a value in seconds is being provided in both cases i.e 30 days ( 2592000 seconds ) and 31 days ( 2678400 seconds). As that sounds like the case, then Couchbase Server is behaving correctly. The expiry value behaves differently when the value provided is below 2592000 then it’s used as a delta in seconds if the value is great, it’s used as a unix epoch. In this case 2678400 becomes 1970-02-01T00:00:00Z. As this is in the past the document is expired.
This is covered in the documentation