Change Time Portion Only Of A DateTime Field With Case Statement - sql-server-2008

Question

I am wanting to use a case statement to change the time piece only of a datetime field in SQL Server 2008 I have a query that will change the time piece, but I need to know how to keep the date portion intact so only the time piece is altered. Meaning if the datetime is
01/01/2015 08:45:10.863
with my syntax I would want to alter it to
01/01/2015 08:30:00.000
This is my syntax which as I said will change the time portion but it does not retain the date. How can I keep the date and change the time portion only?
Create Table #Test
(
[charactername] varchar(100)
,[lefttabletime] datetime
)
Insert Into #Test Values
('Bob Goblin', '01/01/2015 08:14:23.000'),
('Grab Crab', '01/01/2015 08:30:56.023'),
('Mike Knight', '01/01/2015 08:45:10.863')
Select
[charactername]
,case when CAST([lefttabletime] As TIME) > '08:40:00.000' THEN '08:30:00.000'
else [lefttabletime]
FROM #Test
Drop Table #Test
EDIT Additional Syntax Attempted
This threw an error of:
Msg 241, Level 16, State 1, Line 10
Conversion failed when converting date and/or time from character string.
And I tried this syntax
Select
[charactername]
,case when CAST([lefttabletime] As TIME) > '08:40:00.000'
THEN CAST(CAST(CONVERT(DATE, [lefttabletime],101) AS VARCHAR)
+ '08:40:00.000' AS DATETIME) else [lefttabletime] end
FROM #Test

Answer 1

I find it easier if you split the leftabletime into a date and a time component first, the recombine them with the new time portion:
;WITH cte AS
(
SELECT charactername,
[date] = CAST(lefttabletime as date),
[time] = CAST(lefttabletime as time)
FROM #test
)
SELECT charactername,
CAST([date] as datetime)
+ CAST(CASE WHEN [time] > '08:40:00' THEN '08:30:00' ELSE [time] END as datetime)
FROM cte
You can merge the two statement together but I like the simplicity and clarity that a separate CTE provides.

Answer 2

I usually try to make datetime rounding problems fit the DATEADD/DATEDIFF pattern, and I've managed to do that here:
Create Table #Test
(
[charactername] varchar(100)
,[lefttabletime] datetime
)
Insert Into #Test Values
('Bob Goblin', '2015-01-01T08:14:23.000'),
('Grab Crab', '2015-01-01T08:30:56.023'),
('Mike Knight', '2015-01-01T08:45:10.863')
select charactername,
CASE WHEN DATEDIFF(minute,'20010101',lefttabletime)%60 >= 40 THEN
DATEADD(minute,((DATEDIFF(minute,'20010101',lefttabletime)/30)*30),'20010101')
ELSE lefttabletime END
from #Test
Drop Table #Test
The expression DATEADD(minute,((DATEDIFF(minute,'20010101',lefttabletime)/30)*30),'20010101') rounds the time of a datetime down to the nearest 30 minutes (20010101 is an arbitrary date and doesn't need to be adjusted in any way).
I also just use a separate DATEDIFF to find the required matching condition. Where possible, with datetime data, I try to keep it in datetime variables, or, at worst, ints. As soon as you convert to strings you have to start worrying about formats, etc, which I'd usually rather avoid.
Result:
charactername
--------------------- -----------------------
Bob Goblin 2015-01-01 08:14:23.000
Grab Crab 2015-01-01 08:30:56.023
Mike Knight 2015-01-01 08:30:00.000