Reporting 1969 as the last login date for active users

Warning email body is reporting 1969 as the last login date for some users that are actually active:

Hello [edited],

We haven't seen you at [edited] since Dec 31, 1969,
and we miss you! This automatic message is to warn you that your account will
be disabled in 2 weeks unless you come back and visit us before that time.

Please visit us at http://[edited].

Sincerely,
[edited]

I haven't tried to reproduce it yet, but that was forwarded back to me from a user on a live site.