Time::Local has the concept of a "rolling century" which is
defined at 50 years on either side of the current year. Since
it's now 2020 and >50 years since the Unix epoch, the year "70"
gets interpreted by Time::Local as 2070-01-01 instead of
1970-01-01.
Since NNTP servers are unlikely to store messages from the
future, we'll feed 4-digit year to Time::Local::{timegm,timelocal}
and hopefully not have to worry about things until Y10K.
This fixes test failures on t/v2writable.t and t/nntpd.t since
2020-01-01.