I would submit to you in regards to a Rapture "watch date" it does not matter.  Either date could be correct and the other the "mirror" reflection of prophetic fulfillment.  God ordered Moses to make Nissan the 1st and clearly He knows how all things will play out.  So either God did this to order things correctly, or set up the prophetic mirror image time.

Here is a Chabad link showing Rabbi's have differing opinions....

http://www.chabad.org/calendar/view/day_cdo/aid/211011/jewish/Great-Flood-Begins.htm

 

Maranatha,

Tony