I don't know if this forum is the correct place to post this warning. Anyway the Julian date calculator available in the AAVSO site is buggy.
For example 1st January 1 12:00:00 UT is computed as 1721426.00000, the correct answer should be 1721424.00000 according to
Naval Oceanography Portal http://www.usno.navy .mil/USNO/astronomical-applications/data-services/cal-to-jd-conv
Institute de Mecanique Celeste http://www.imcce.fr/en/grandpublic/temps/jour_julien.php
Moreover 1st Jan -4712 (astronomical year) 12:00:00 is computed as 39.00000 instead of 0 (zero).
For dates useful for observation reporting (i.e. close to 2011) it seems working but the AVVSO calculator is offered as a generic tool and should (IMHO) work for every date.
Hope this helps