Currently krb5_timestamp is 32-bits. Changing the prototypes to
krb5_timestamp will make it more obvious what the impact will be when
we redefine krb5_timestamp as a 64-bit value. I believe krb5_timestamp
should be defined in terms of time_t.
Reading RFC 4120 I do not believe there are any bit length restrictions
on the number of seconds represented in a KerberosTime. Therefore, we
should attempt to remove the restrictions in our own implementation.