OOP: When can it be sold?

[ecaplan_52556]

My local bookseller (Iowa, USA) insists that they cannot sell OOP
before June 21, 12:01 Central Daylight Savings Time.  But I
thought that the restriction was no sales before 12:01 British
Summer Time.  Doubly unfortunate for me, I live in a small
town and none of the stores will be open past midnight, so I
need to wait until the next morning to buy, 14-16 hours after
release!

If it is BST, then I should be able to buy OOP at June 20, 18:01
in Iowa, USA... Right?

*IF* right, then is there an official source for this information
that I can use to persuade my bookseller?

Thanks much.

[I really apologize if this is redundant.  I've looked through the
archives and searched the web and haven't been able to find
the answer.]