This is a Vienna-derived virus, virus, but it marks infected files by
setting the "month" field to 13, instead of setting the "seconds" field to
62. W-13 has been reported to have originated in Russia as well as in
Poland.
A 507 bytes variant with some bugs corrected is also known.
This is a minor modification of the 507 byte variant mentioned above. It
will set the "month" field to 12, which makes it less obvious, as 12 is a
legal value, but this also makes all files created in December immune to
infection.