Sometimes you want to compare a datetime value without the time portion to a date. If you use query param and specify a timestamp for the type but only provide a date as the value you'll be given 00:00:00 for the time, which is midnight. To strip, or more specifically, set the time porition of the value in the database or midnight also use this cast:

CAST(FLOOR(CAST(DateCompleted AS FLOAT)) AS DATETIME)

What this will do is convert the date to a floating point number, remove the decimal, then flip it back to a datetime data type. When the datetime data type is represented as a floating point number the whole number is the date portion and the decimal portion represents the time. Flooring removes the decimal and effectively set it to midnight.