You should use the english date format 'mm/dd/yyyy' and enclose the date specifications by hash characters.
With ranges, use the
BETWEEN
operator and include the times:
SELECT * from Customer WHERE RegDateTime BETWEEN #02/15/2012 00:00:00# AND #02/25/2012 23:59:59#
If you want to limit the query to a single day, use the
LIKE
operator:
SELECT * from Customer WHERE RegDateTime LIKE #02/15/2012#