On the BDP download summary page we offer a Manual Installation package for 64 bit operating systems. See http://www.basicdatepicker.com/download/
If you swap out your existing .dll with the x64 version everything should work fine.
Hope this helps solve the problem. Please feel free to contact us again in the future if you have any questions or comments and we will do our best to help out.
Thanks,
Geoffrey McGill - Product Manager
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Coolite Inc. - ASP.NET WebControls.
Email: support [at] coolite [dot] com
Phone: +1(888)775-5888