DateTime Efficiency and Best Practices Question

I use this convert, float, and floor things a lot, and I'll admit that sql is probably not my strongest skill.  It seems like I remember starting doing this when I got invalid results, but someone told me that it was very inefficient, and I do a lot of date/time based queries in this app, and I really need to get it right.  

Here are some examples:
-- Gets it based on certain dates... say all orders from January 10th to 13th
Select avg(datediff(second,w.dispBeginTime,w.dispFinishTime))
  From WorkOrder w
  Where      w.keyCompany = @keyCompany
      and      w.dispBeginTime  is not null
      and w.dispFinishTime is not null
      and floor(convert(float,w.dispFinishTime)) >= floor(convert(float,@BeginDate))
      and floor(convert(float,w.dispFinishTime)) <= floor(convert(float,@EndDate))

-- Gets it based on certain times... say all orders from January 10th 8:01:01 to January 13th 8:02:02
Select avg(datediff(second,w.dispBeginTime,w.dispFinishTime))
  From WorkOrder w
  Where      w.keyCompany = @keyCompany
      and      w.dispBeginTime  is not null
      and w.dispFinishTime is not null
      and convert(float,w.dispFinishTime) >= convert(float,@BeginDate)
      and convert(float,w.dispFinishTime) <= convert(float,@EndDate)

Thanks, Tony
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Anthony PerkinsCommented:
Try it this way:

Select @BeginDate = CAST(CONVERT(char(8), @BeginDate, 112) as datetime),      -- Or smalldatetime
          @EndDate = CAST(CONVERT(char(8), @EndDate, 112) as datetime)

Select ...
Where ...

 and CONVERT(char(8), w.dispFinishTime, 112) BETWEEN @BeginDate And @EndDate

It is still not going to be able to take advantage of any index, but it should be better.

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@acperkins: Have you made any tests? I did some test yesterday (I tried three methods of cutting off the time: with CONVERT, with FLOOR, and  with DATEADD functions). And it seems to me like their performance is very similar (or even the same). But if you have any test results available or maybe you can provide some link to some reliable results of someone else I would be grateful.
picsnetAuthor Commented:
i did something similar where i used the between, but i kept my dates in floats....
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