Here is a little trick that will "strip" (In reality zero out) the date or time portion from a datetime value. Basically the SQL datetime is just two 4 byte integers. When you convert a datetime to a float the integer portion represents the date (Which is the number of days since 1/1/1900) and the fractional portion represents the time (The number of 3 millisecond increments that have elapsed since midnight). To zero out one or the other simply requires removing the integer or fractional portion then converting that back into a datetime. The following illustrates this:

DECLARE @DateTime float


/* Date & Time */
SELECT CAST(@DateTime AS datetime) AS DateAndTime

/* Date only */
SELECT CAST(FLOOR(@DateTime) AS datetime) AS DateOnly

/* Time only */
SELECT CAST((@DateTime - FLOOR(@DateTime)) AS datetime) AS TimeOnly

I think this is much cleaner than the common method of converting the datetime to a varchar, truncating the date or time portion, then converting the varchar back to a datetime. You can read more about the datetime data type here in the BOL.