I agree with translating UINT64_MAX to mean "nil Date", but there are still corner cases here.

The error was:

Double value cannot be converted to UInt64 because the result would be greater than UInt64.max

It appears Swift is more strict when casting a double to an int that normal C is. Normal C will gladly cast between double and uint64_t, even if the double value is INFINITY. But Swift does this:

/// If source is outside the bounds of this type after rounding toward
/// zero, a runtime error may occur.

So this code in Credentials.swift can still explode:
expirationTimeout = UInt64(expiration.timeIntervalSince1970)