I suggest that BMSN management will wait until at least 3 days AFTER that date to ensure no glitches with settlement of funds, etc. Then they will access records for those who held shares on the 18th. What happens on the 19th is not relevant...if you held on the 18th and sold all on the 19th...you get the divy for the shares held on the 18th.
In all honesty, if something can go wrong with that position, it will....that is only the way I understand it should work.