clock scan strangeness

I was wondering why clock scan works as below:
% clock scan z
1219336516
% clock scan zz
unable to convert date-time string "zz"

and also
%clock scan ""
1219347325

it works like this with any letter. I saw that in 8.4 it was a bit
different always giving 1219266000.
what is the hidden cause for this behaviour?
.