Datetime_select and preserving NULLs

I have several datetime columns and in my ‘new’ and ‘edit’ views
sometimes I would like to set a specific date time while other times I
would like to leave it null. For the life of me I can’t figure out what
I should be using in my _forms.rhtml so that datetime_select accepts a
NULL value for the datetime instead of a preset value. I’ve tried
setting the default value of the column to NULL or ‘0000-00-00
00:00:00’ but to no avail.

I see there is an open defect on this issue:
http://dev.rubyonrails.org/ticket/2999 but was wondering if there was
an easy work around that someone could recommend.

Thanks,
Matt