When integrating Mender with my C.H.I.P. board, I came across the Certificate expired or not yet valid error. I adjusted the time and date on my C.H.I.P., but Mender would still spit out this error.
I later figured out that I was missing the ca-certificates package on my board and my certificate chain was untrusted. Installing root CA certificates solved the issue.
However, I file this (low priority) bug report since the Mender log output is clearly wrong.