X509Certificate2.Verify() with moved clock returns false

Question

As far as I understand, my certificate should be valid at any time, as long as it was signed before April 2014 (while signing, we do put timestamp).

If I calling X509Certificate2.Verify() on my signed assembly, it returns true.

However, if I move my computer's clock forward, to say 2019 - calling X509Certificate2.Verify() returns false. This is unexpected, since I thought it still should be valid.

Could anyone explain me this behavior?

Thanks