Stored Proc Bug With Datetime Variable

two variables declared in my proc:
@DATE_RANGE_START as datetime,
@DATE_RANGE_END as datetime,

When I execute my SP it takes 34 seconds.

When I change the variables to:
@DATE_RANGE_START1 as datetime,
@DATE_RANGE_END1 as datetime,

and add this to my sp:
declare @DATE_RANGE_START datetime
declare @DATE_RANGE_END datetime
set @DATE_RANGE_START = @DATE_RANGE_START1
set @DATE_RANGE_END = @DATE_RANGE_END1
the SP runs in 9 seconds (which is expected)

Passing in '1/1/01' and '1/1/07' respectivly.

Everything else is equal and non-important to this problem.
Why does it take 34 seconds when I use the variables from the input
parameters?
Interesting isn't it.
Jeff