the value of AuthnInstant

Suppose a user presents an AuthnRequest to an IdP at time t0.  Since
the user has no security context initially, the IdP challenges the
user to authenticate, which the user does successfully (at time t1).
So the IdP issues an assertion (at time t2), which the user transmits
to the SP via the browser.

At some later time t3, the user presents another AuthnRequest to the
IdP.  Since the user already has a security context, the IdP does not
challenge the user to authenticate, but rather issues an assertion (at
time t4), which again the user transmits to the SP.

Question: What is the value of AuthnInstant in the second assertion?

Thanks,
Tom